Added SETAM, set the audio Mixer command to the supported EX-Rail commands.
the audio mixer can now be set at startup by using the configuration in myHall.cpp and it can be modified in an EX-Rail scripts.
Syntax: PLAYSOUND(<vPin>, 0, <am>, SETAM)
Valid values for <am>: 1 or 2
Completed implementation of exposing most relevant DFPlayer commands
Fixed a problem with RX corrupted data from DFPlayer.
Added retry (currently max retry 3) after timeout and recovery
Note: Need to think of a more elegant way to recover from a reset command
