Fix pjrc/usb_debug_only for 8MHz devices.
Add benito for m32u4 code.

Use PJRC's usb_serial code (modified to run with 8MHz crystal) to write
a simple USB-to-serial converter. DTR pulls the reset line low
(autoreset). Still won't work with UNO, probably because on 8MHz boards
the closest rate to 115200 is still too far.