yaw improvement

this should stop the copter from carrying on spinning at high yaw rates.
allow controlling external XOR gate connected to PB2 (BOOT1) to invert USART2_RX signal on F1. Currently used by sbus driver.
On second thought, no need to make inverter configurable. only sbus will use it, and the driver can set inversion automatically.
Fixing gimbal_flags not working when feature SERVO_TILT was enabled. This was done by properly calculating servo offset instead of depending on enabled features/mixers.