yaw improvement

this should stop the copter from carrying on spinning at high yaw rates.
Merge pull request #120 from luggi/yaw

yaw improvement
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.
new Baseflight PID

full gyro scale is used now
and a new pid with float calculations and proper units is added based on PIDrewrite
eeprom size is also increased from 1kB to 2kB

Conflicts:
	src/config.c
	src/imu.c