mbed: Implement host communication over USB/serial

Can now be reprogrammed live from Flowhub!
runtime: Use an enum for pullup config

Microcontrollers other than Atmel AVR can have other pullup modes
than just None and Up, so prepare for supporting this.
components: Fix LedMatrixMax truncating pin identifiers

Caused it to not work on platforms using more than 8bit for
pin ids, like the Mbed.
mbed: Implement pin and pullup mode