Introduce Rust FSBL

This PR introduces some major features while also changing the project structure to be more flexible
for multiple platforms (e.g. host tooling).

Added features:

1. Pure Rust FSBL for the Zedboard. This first variant is simplistic. It
   is currently only capable of QSPI boot. It searches for a bitstream
   and ELF file inside the boot binary, flashes them and jumps to them.
2. DDR, QSPI, private CPU timer modules
3. Tooling to auto-generate board specific DDR and DDRIOB config
   parameters from the vendor provided ps7init.tcl file

Changed project structure:

1. All target specific project are inside a dedicated workspace inside
   the `zynq` folder now.
2. All tool intended to be run on a host are inside a `tools` workspace
3. All other common projects are at the project root

scripts/memory_ddr.x

@@ -0,0 +1,22 @@
+MEMORY
+{
+  /* Zedboard: 512 MB DDR3. Only use 63 MB for now, should be plenty for a bare-metal app.
+  Leave 1 MB of memory which will be configured as uncached device memory by the MMU. This is
+  recommended for something like DMA descriptors. */
+  CODE(rx) : ORIGIN = 0x00100000, LENGTH = 63M
+  UNCACHED(rx): ORIGIN = 0x4000000, LENGTH = 1M
+}
+
+REGION_ALIAS("DATA", CODE);
+
+SECTIONS
+{
+  /* Uncached memory */
+  .uncached (NOLOAD) : ALIGN(4) {
+    . = ALIGN(4);
+    _sbss_uncached = .;
+    *(.uncached .uncached.*);
+    . = ALIGN(4);
+    _ebss_uncached = .;
+  } > UNCACHED
+}