Bootloader infrastructure and main logic (#404)

* adds bootloader code without a SHA256 implementation

* small fixes and typos

bootloader/README.md

@@ -0,0 +1,36 @@
+# OpenSK Bootloader
+
+This bootloader supports upgradability for OpenSK. Its functionality is to
+
+-   check images on A/B partitions,
+-   boot the most recent valid partition.
+
+## How to use
+
+The bootloader is built and deployed by OpenSK's `deploy.py`. If your board
+defines a metadata address, it is detected as an upgradable board and this
+bootloader is flashed to memory address 0.
+
+## How to debug
+
+The bootloader prints debug message over RTT when compiled in debug mode. Using
+`nrfjprog` for flashing and inspecting memory is recommended for debugging.
+
+```shell
+RUSTFLAGS="-C link-arg=-Wl,-Tlink.x -C link-arg=-nostartfiles" \
+    cargo build --target thumbv7em-none-eabi
+llvm-objcopy -O ihex target/thumbv7em-none-eabi/debug/bootloader \
+    target/thumbv7em-none-eabi/debug/bootloader.hex
+nrfjprog --program target/thumbv7em-none-eabi/debug/bootloader.hex \
+    --sectorerase -f nrf52 --reset
+```
+
+To read the debug messages, open two terminals for:
+
+```shell
+JLinkRTTLogger -device NRF52840_XXAA -if swd -speed 1000 -RTTchannel 0
+JLinkRTTClient
+```
+
+The first command also logs the output to a file. The second shows all output in
+real time.