Stop supporting 8-bit AVR (AtMega328)

[laurb9]

The effort and code complexity for continuing to support AtMega328 and friends are not worth it and could be better invested elsewhere.

I see no good reason not to move to 32bit platforms. Teensy LC is an Arduino-compatible 32bit ARM board close to the official Arduino Pro Mini price point.

Advantages

• get rid of PROGMEM hacks for split data/code memory, use const as intended
• more freedom to choose add-on boards (displays, etc)
• no more bumping into 2K memory limit (can revisit menu definition)
• get rid of 16-bit integer problems

Disadvantages

• some Pro Mini clones are cheap but I'm not trying to save pennies.

[laurb9]

Current official target board is the $12 Teensy LC: ARM Cortex-M0+ processor at 48 MHz, 62K Flash, 8K RAM
Will look also into Adafruit Feather Cortex-M0 solutions.

[laurb9]

Done in v1.2