Can step through available frequencies for bands / channels. Implements feature suggested by issue betaflight#33.

If Cleanflight/Betaflight has 'SetFreqByMHzMsp' support then any frequency in MHz can be set (via band 'U'); see: http://www.etheli.com/CF/vtxCfgCMSFreqViaMsp

v0.41 Changes:

When doing a 'save' on the VTX screen, now performs an EEPROM_SAVE after updating the values (needed for latest version of Betaflight)

Shows 'Retrying' if save-retry happens

For full list of changes see commit entry here

--ET