SPI:
- use PDC DMA for SPI transfers

RC632:
- Fix locking between 'atomic' ops like set/clear bit and RC632 IRQ
- Implement VFIFO handling

USB:
- Implement suspend/resume handshake
- Make sure we don't eat a lot of power before entering the configured state

DFU:
- add a header to the application program, checksum and size
- checksum application image.  If wrong, enter DFU recovery mode
- Get rid of lots of constant values and replace them by proper defines

Way Later, for librfid:
- Implement parts (or all of) 14443 in firmware as alternative configuration