SERVO_BLH_RVMASK is unintuitive to set

[rmackay9]

The SERVO_BLH_RVMASK parameter is apparently difficult for users to know how to set. We have these opinions on the parameter:

Opinion 1: the bits seem to have no relation to the motor or servo output numbers
Opinion 2: the bits should match the servo output number

The setting is probably different based on whether the autopilot has an IOMCU or not

This enhancement request was raised during the 4.6 beta testing