TELEMETRY_MOD=14051 (by gerardv @ forum.turnigy9xr.com) makes USART0 available
by reallocating AIL and THR onto pins 14 and 15 of the 14051 multiplexer.
TELEMETRY_MOD=14051_SWAPPED is based on TELEMETRY_MOD=14051, but uses USART1
instead of USART0, freeing USART0 for other purposes (debugging, interfacing
via AVRISP port). USART1 is freed up by reallocating LVert Trim Dn/Up onto
14051's 12 and 1 pins respectively.
Additionally, this commit cleans a few things up:
* moves AVR ADC-related code to adc_driver.cpp,
* provides #defines to have a convenient way to access either USART0 or USART1
registers depending on static (build-time) configuration
* moves code related to this telemetry mod to board_stock
* replaces a few magic numbers with enums.
Thanks to Bernard Songis for review and suggestions!
JivaroFAD