added full crosstool-ng instructions

doc/README-rpi.md

@@ -440,13 +440,14 @@ scp <user_name>@<ip-address>:/lib/arm-linux-gnueabihf/libc.so.6 <rootfs_path>/li
 If any custom libraries are used which rely on symlinks, it might be necessary to copy them
 or create them manually as well.
 
-# <a id="ctng"></a> Building a cross-compile toolchain with crosstool-ng
+# <a id="ctng"></a> Building a cross-compile toolchain with `crosstool-ng`
 
 If you want to cross-compile the toolchain for the Raspberry Pi, you can do so
 with the `crosstool-ng` tool.
 
 Alternatively, you can also download a cross-compile toolchain built with
-crosstool-ng for the Raspberry Pi 3 [from here](https://www.dropbox.com/sh/zjaex4wlv5kcm6q/AAABBFfmZSRZ7GE7ok-7vTE6a?dl=0)
+crosstool-ng for the Raspberry Pi 3 and the Raspbery Pi 4 
+[from here](https://www.dropbox.com/sh/zjaex4wlv5kcm6q/AAABBFfmZSRZ7GE7ok-7vTE6a?dl=0)
 inside the `x-tools` folder.
 
 ## Ubuntu
@@ -474,8 +475,8 @@ inside the `x-tools` folder.
    sudo make install
    ```
 
-   You can also install `ct-ng` locally by supplying `--prefix=<installPath>` to the `configure` 
-   command. You don't need `sudo` for the `make install` command if you do this
+   You can also install `ct-ng` locally by replacing `--enable-local` with `--prefix=<installPath>`
+   to the `configure`  command. You don't need `sudo` for the `make install` command if you do this.
 
 4. You can get a list of architectures by running 
 
@@ -506,7 +507,20 @@ inside the `x-tools` folder.
    It is recommended to go to `Paths and misc options` and disable `Paths and misc options`.
    Remember to save the configuration.
 
-5. Finally, after finishing the configuration you can build the toolchain with
+5. The correct configuration files for the Raspberry Pi 3 and Raspberry Pi 4 are provided here
+   are were set up according to [these instructions](https://medium.com/@stonepreston/how-to-cross-compile-a-cmake-c-application-for-the-raspberry-pi-4-on-ubuntu-20-04-bac6735d36df).
+   You can download those files from [here](https://www.dropbox.com/sh/ok554i453phuqec/AACcv-fxpIXi3Nh1z8UWu4Sea?dl=0).
+
+   Put the `patches` folder, the `.config` folder and the `env.sh` in the same directory where
+   you run `ct-ng`. After that, you can check the configuration with `./ct-ng menuconfig`.
+
+   Set up the multiarch environment for debian by running
+
+   ```sh
+   . env.sh
+   ```
+
+6. Finally, after finishing the configuration you can build the toolchain with
 
    ```sh
    ./ct-ng build
@@ -514,3 +528,17 @@ inside the `x-tools` folder.
 
    This takes 20-30 minutes. You can find the toolchain in the `~/x-tools` folder after building
    has finished
+
+7. You can check whether the toolchain was configured correctly for multiarch by running the
+   following command
+
+   ```sh
+   cd ~/x-tools/armv8-rpi3-linux-gnueabihf/armv8-rpi3-linux-gnueabihf/bin
+   ./ld --verbose | grep -i "search"
+   ```
+
+   The output should include
+
+   ```sh
+   SEARCH_DIR("=/lib/arm-linux-gnueabihf"); SEARCH_DIR("=/usr/lib/arm-linux-gnueabihf");
+   ```