OpenBLT-based USB serial bootloader

[mck1117]

• Avoids DFU driver woes by presenting an identical serial interface to the one used while the ECU is running
• Create an OpenBLT target called "chibios". Runs OpenBLT as an app atop ChibiOS, instead of running it as its own port on the bare metal. This lets us recycle the existing FOME configuration of ChibiOS, its HAL, and critically the USB serial driver.
• Gut the previous stuff in firmware/bootloader, and replace it with the OpenBLT-on-ChibiOS bootloader.
• Delete board-specific openblt stuff.