implement 3wb bootloading

[twatteyne]

input from other repos:

• Teensy code that implements the "bit wiggling:" https://github.com/PisterLab/scum-test-code/blob/fil/scm_v3c/teensy_uC_programmer/teensy_uC_programmer.ino
• Python script that sends the binary image to Teensy over UART: https://github.com/PisterLab/scum-test-code/blob/austin/scm_v3c/bootload/bootload.py
• Marks' nRF52840-DK code: https://github.com/PisterLab/scum_programmer

[twatteyne]

commands we want to send from the computer to the programmer:

computer -> nRF message types

• send a byte over UART to SCuM
• loading new SCuM binary from the computer into the RAM memory of the nRF (64kB); there is 256 kB of RAM in the nRF52840
• trigger a 3wb bootload sequence

nRF -> computer message types

• received a byte over UART from SCuM
• operation completed