Demo STM32L0 support

[karlp]

Just a demo really of targetting an M0+ part.

I don't actually have any use for this going forward, but I had 99% of the code done, so tested it at least, and thought I'd share it here if anyone else wants this sort of thing.

[blakesmith]

@karlp Thanks for sharing this! I'm working on a project that uses an STM32L053. I was surprised that the chip has a USB peripheral, but the built-in bootloader from STM can't do USB DFU like other STM parts I've used (STM32F042 for example).

Going to give this a try for the bootloader instead!