ADC mutex

[ginge]

In snowy, the ADC is used for multiple things, including ambient light and battery level.

There is no mutex protecting this resource. In fact, there is no common agreed way to talk to ADC

• Create mutex in suitable rebbleos file.
• implement in ambient and power

Additionally:

• ADC init has a "commoninit" for the ADCs as a whole. currently this init lives in snowy_power.c. It should be in main.c/adc.c
• ADC init is done by each hw thread. ambient will init with it's preferred setting, then adc will do the same. Change both drivers to do a full re-init of their preferred settings and sample times before using the module.
• re-enable ambient

[AnotherOctopus]

So how I interpreted implementing this is bulbing out _adc_init and _read_adc into a new file called snowy_adc.c/h in the snowy directory, and then implementing a new file called adc.c in rcore, which handles the calls to the hardware and handles the mutex. Then an ambient.c and an implemented battery.c make calls to the adc.c for hardware access.