reorganization of uart-based receiver drivers
FEATURE_SPEKTRUM has been removed and replaced with FEATURE_SERIALRX.
cli  option serialrx_type now configures what type of receiver it is
0 = spektrum1024, 1 = spektrum2048, 2 = sbus
sbus will need hardware inverter to use.
also cleaned up receiver drivers to assign readrawRC callback instead of assigning in code in main()
none of this has been tested.

git-svn-id: https://afrodevices.googlecode.com/svn/trunk/baseflight@418 7c89a4a9-59b9-e629-4cfe-3a2d53b20e61