The primary reason is to support the D4R-II with it's much faster PWM
frequency. The PWM RSSI code could not keep up, and since there are no
timers free for using capture compare of PWM signals in hardware one
solution is to use the ADC at a slow sample rate.
RC2 is used as before and it expects a signal between 0 and 3.3v. An
inline smoothing capacitor may help.
This commit also removes the cli command adc_power_channel since the
reading was never actually exposed anywhere.
Also support FrSky 1khz RSSI. See documentation also added in this
commit.
This commit also cleans up the PWM mapping code. 'mask' didn't need to
be a mask and it wasn't possible to add another 'type' since there were
only 4 possible values when it was a mask and they were already defined.
Combined with switching to using 16 bits instead of 8 for the mapping
configurations, it's now possible to have 256 types instead of 4 at the
expense of a few bytes of flash.
Moved the RSSI calculation into rx_common.c, previously it was in the
main loop.
