STM32L4-based motor controller

I built some motor controller hardware, and I wanted to use an open-source toolchain to make it go.

Build and debug prerequisites

scons
gcc-arm-none-eabi
gdb-arm-none-eabi
stlink

Shoutouts

xpcc, which taught me a great deal about SCons in an embedded context.
Seth LaForge, from whom I liberally stole compiler flags.