fnet_stm32_eth.c: Mostly ported. TODO: Chibi to FNET descriptors in I/O funcs.
fnet_stm32_serial.c: Ported. Untested.
fnet_chibios.c: Added FNET_CFG_OS_EVENT section. TODO: Test the CMSIS ISR mech
TODO: IRQ ISR stuff in fnet_stm32.c. Stubs there now. Might be better in
OS-specific code side.