Relocate common code which can be used by drivers and by main into
'common'.  Cleanup includes.  Fix FY90Q target compilation.
Fixing and unifying Serial RX initialisation.  Destination arguments now
go last and each init method is passed the rxConfig.
Cleanup declarations of failsafeCnt so that it is only in failsafe.h and
failsafe.c.  Now it is clear from the list of includes that each
serialrx provider supports failsafe.