[ahrs] Upgraded GX3 ahrs subsystem

1) Separate makefiles for fixedwing and rotorcraft (so it can be used in both)
2) Configurable GX3 initialization
3) Acceleration and rates properly rotated into the body frame
4) uses float imu struct to keep the precision
5) adds GX3_INFO message
6) fixed IMU_GYRO and IMU_ACCEL messages for float imu struct
7) removed ahrs_extern_quat interface as obsolete

Especially the fixed point imu struct is used for backwards compatibility,
maybe there is a more elegant way how to use the floating point interface.

after review:

- changed UART macros for uart functions
- GX3 variables moved into ahrs struct
- rates and accel used in imu frame and stored in imu.f
- removed unnecessary math macros
- renamed variables to lowercase and removed unused ones
- removed RMAT to Quat conversion,
- using strictly RMAT and Eulers (if needed)
- better written calculation of gx3 frequency. It is actually more precise like this.

closes pull request #504