readme bbb update for linux and windows

doc/README-bbb.md

@@ -68,6 +68,87 @@ These prerequisites are valid for Linux as well as Windows hosts.
 ## Cross-Compiling on a Linux Host
 
 ### Setting up prerequisites for cross-compiling
+
+1. Install `CMake` and `rsync`
+
+   ```
+   sudo apt-get install cmake rsync
+   ```
+
+2. Configure the Beagle Bone Black linux environment. The last section of the
+   [Linux REAMDE](README-linux.md#top) specifies how to set up a UNIX environment
+   for the FSFW and isalso applicable to the Raspberry Pi. SSH into the
+   Beagle Bone Black and follow the instructions in that section.
+
+3. Install the correct [ARM Linux cross-compile toolchain](https://releases.linaro.org/components/toolchain/binaries/latest-7/arm-linux-gnueabihf/).
+   provided by Linaro.
+
+   Test the toolchain by running:
+
+   ```sh
+   arm-linux-gnueabihf-gcc --version
+   ```
+
+4. Set up a sysroot folder on the local host machine. Make sure the SSH connection to
+   the BBB is working without issues. Then perform the following steps
+
+   ```sh
+   cd $HOME
+   mkdir beaglebone
+   cd beaglebone
+   mkdir rootfs
+   cd rootfs
+   pwd
+   ```
+
+   Store the result of `pwd`, it is going to be used by `rsync` later.
+
+   Now use `rsync` to clone the BBB sysroot to the local host machine.
+   You can replace `<ip-address>` with `beaglebone.local` to use DNS.
+   Use the rootfs location stored from the previous steps as `<rootfs-path>`.
+
+   ```sh
+   rsync -avHAXR --delete-after --info=progress2 --numeric-ids <user_name>@<ip_address>:/{usr,lib} <rootfs_path>
+   ```
+
+   On Linux, it is recommended to repair some symlinks which can be problematic:
+   Navigate to the folder containing the symlinks first:
+
+   ```sh
+   cd <rootfs_path>/usr/lib/arm-linux-gnueabihf
+   ```
+
+   You can now use
+
+   ```sh
+   readlink libpthread.so
+   ```
+
+   which will show an absolute location of a shared library the symlinks points to. This location
+   needs to be converted into a relative path.
+
+   Run the following command to create a relative symlinks instead of an absolute ones. The pointed
+   to location might change to check it with `readlink` first before removing the symlinks:
+
+   ```sh
+   rm libpthread.so
+   rm librt.so
+   ln -s ../../../lib/arm-linux-gnueabihf/libpthread.so.0 libpthread.so
+   ln -s ../../../lib/arm-linux-gnueabihf/librt.so.1 librt.so
+   ```
+
+   For more information on issues which can occur when cloning the root filesystem,
+   see the [troubleshooting](#troubleshooting) section.
+
+5. It is recommended to install `gdb-multiarch`. This tool will allow remote debugging on the host
+   computer. This is not required if the `tcf-agent` is used.
+
+   ```sh
+   sudo apt-get install multiarch
+   ```
+
+6. Perform the steps [in the following chapter](#cross-test) to build the
+   software for the BBB and test it.
    
 ## Cross-Compiling on a Windows Host
 
@@ -126,15 +207,15 @@ These prerequisites are valid for Linux as well as Windows hosts.
    
    Please note that `rsync` sometimes does not copy shared libraries or symlinks properly,
    which might result in errors when cross-compiling and cross-linking. It is recommended to run
-   the following commands in addition to the `rsync` command:
+   the following commands in addition to the `rsync` command on Windows:
    
    ```sh
    scp <user_name>@<ip-address>:/lib/arm-linux-gnueabihf/{libc.so.6,ld-linux-armhf.so.3,libm.so.6} <rootfs_path>/lib/arm-linux-gnueabihf
    scp <user_name>@<ip-address>:/usr/lib/arm-linux-gnueabihf/{libpthread.so,libc.so,librt.so} <rootfs_path>/usr/lib/arm-linux-gnueabihf
    ```
    
-   See the [troubleshooting](#troubleshooting) section for more details if the compilation process
+   For more information on issues which can occur when cloning the root filesystem,
-   is problematic.
+   see the [troubleshooting](#troubleshooting) section.
 
 5. It is recommended to install `gdb-multiarch`. This tool will allow remote debugging on the host 
    computer. Replace `x86_64` with the correct processor architecture for other architectures.
@@ -281,25 +362,36 @@ There might be some issues with the pthread symbolic links.
 Navigate to the folder containing the symlinks
 
 ```sh
-cd /c/User/<UserName>/beaglebone/rootfs/usr/lib/arm-linux-gnueabihf
+cd <rootfs_path>/usr/lib/arm-linux-gnueabihf
 ```
 
-Type `more libpthread`, press  `TAB` and check whether the symbolic 
+ Type `more libpthread`, press  `TAB` and check whether the symbolic 
-link `libpthread.so` is shown. If it is not, we are going to set it up
+ link `libpthread.so` is shown. If it is not, we are going to set it up
-manually to avoid issues when linking against `pthread` later. 
+ manually to avoid issues when linking against `pthread` later. 
+ Now you can find out where `libpthread.so` points with `readlink libpthread.so`.
+ This information is used to convert the absolute symlink to relative ones, for example with:
 
-Run the following command to create a symlink to `libpthread.so.0`
+Run the following command to copy the symlink `libpthread.so.0` if it does not exist yet:
 
 ```sh
 scp <user_name>@<ip-address>:/usr/lib/arm-linux-gnueabihf/libpthread.so .
 ```
 
-Alternatively, you can also create the symlink pointing to the correct library with
+Alternatively, you can correct the symlinks to use relative paths, for example with:
 
 ```sh
-ln -s ../../../lib/arm-linux-gnueabihf/libpthread.so.0 libpthread.so 
+ln -s ../../../lib/arm-linux-gnueabihf/libpthread.so.0 libpthread.so
+ln -s ../../../lib/arm-linux-gnueabihf/librt.so.1 librt.so
 ```
-
+ 
+Please note that there might also be issues with some symlinks or libraries not being copied 
+properly, especially on Windows. This has occured with files like `libc.so.6`.
+If there are linker issues at a later stage, you can try to copy the symlinks manually from the
+Linux board to the sysroot with `scp`.
+ 
+For example, you can copy `libc.so.6` from the Linux board to the sysroot with 
+the following command
+ 
 If there are issues with the cross-compilation process, manually copying the following
 symlinks can help: