Add preprocessor v2 code

Based on work from the HK310 expansion project.
Not working yet.
Uses SDCC.
Add .gitignore for SDCC specific files
Remove process-value.c, has been moved into servo_reader.c
Implement HK310 expansion protocol handling
Implement full functionality, but not tested yet
Update TODO list
Update TODO list with MOSFET for switch
Add PDF schematics for those who don't have Eagle installed