refactor makefiles, document how to build

targets/stm32l442/README.md

@@ -0,0 +1,34 @@
+# STM32L4xx Solo
+
+This documents how to build the firmware for Solo for the STM32L4xx microcontroller.
+
+# Building
+
+First build the cbor library.
+
+```bash
+make cbor
+```
+
+Now build the Solo bootloader.
+
+```
+make boot
+```
+
+Now build the Solo application.
+
+```
+make clean      # remove object files from bootloader, keep bootloader.hex
+make
+```
+
+Merge the two files together.  This script also patches a spot in memory to
+allow the bootloader to boot the application.  This memory spot is later used for
+signed firmware updates.
+
+```
+python merge_hex.py solo.hex bootloader.hex all.hex
+```
+
+You can now program Solo with `all.hex`.