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.
Dominic Clifton