add docs on building on linux

docs/building.md

@@ -0,0 +1,28 @@
+To build, develop and debug the firmware for the STM32L442 (WIP!) via cross-compilation on Linux, no vendor-specific software is necessary.
+
+There exists a development board [NUCLEO-L432KC](https://www.st.com/en/evaluation-tools/nucleo-l432kc.html) you can use; the L432 chip differs from the L442 used for Solo only in that it lacks a cryptographic accelerator. The board does contain a debugger, so all you need is a USB cable :)
+
+You will need the following packages (naming given for Arch Linux):
+
+- arm-none-eabi-gcc
+- arm-none-eabi-newlib
+- arm-none-eabi-binutils
+
+and one of
+
+- [openocd](http://openocd.org)
+- [stlink](https://github.com/texane/stlink)
+
+If you remove the `.exe` extensions in the [Makefile](https://github.com/SoloKeysSec/solo/blob/master/targets/stm32l442/Makefile), and possibly add a `-g` flag, compilation runs through.
+
+To flash and step through the code:
+
+* connect the Nucleo to your PC
+* attach one of the debuggers: `st-util` (for stlink), or `openocd -f interface/stlink-v2-1.cfg -f target/stm32l4x.cfg` (for openocd)
+* launch `gdb` via `arm-none-eabi-gdb -q solo.elf`
+* connect gdb to the debugger via `target extended-remote :4242` (for stlink), or `target remote :3333` (for openocd)
+* flash the firmware via `load`
+* optionally set a breakpoint via `break main`
+* `continue`, and start stepping 🙌
+
+Note that the code for `targets/stm32l442` currently consists of only a blinky hello world...

mkdocs.yml

@@ -10,6 +10,7 @@ nav:
     - README.md: repo-readme.md
     - FIDO2 Implementation: fido2-impl.md
     - Signed update process: signed-updates.md
+    - Building: building.md
     - Contributing Code: contributing.md
     - Contributing Docs: documenting.md
     - What the udev?!: udev.md