de-duplicate degrees to radians code.

Note, this also makes the operator precedence clearer.
de-duplicate failsafe code.  extract failsafe code to seperate files.
remove dependency on board.h and mw.h on a few files.

Moved rx configuration paramaters into rxConfig in order to remove the
dependency on config_t from the sbus rx code  - sumd/spektrum to follow.

Avoided use of YAW/PITCH/ROLL in some files since those constants are
from an unrelated enum type.

Replaced some magic numbers with constants to remove comments and
improve code readability.

Note, due to the tight coupling and global variable usage it was
difficult to create a smaller commit.