HAL_Linux: added SBUS decoder from PX4 project

thanks Lorenz!
HAL_Linux: initial support for parallel SBUS and PPM-SUM decoding
Linux_HAL_Essentials: change ring buffer to 300 entries

this ensures full SBUS frames can be decoded
HAL_Linux: change ring buffer to 300 entries
Linux_HAL_Essentials: rebuilt rcinpru0 with 300 ring buffer entries

thanks to Sid for doing this
Copter: Rate Pitch IMAX default to 1000

Spotted by Jonathan Challinger, thanks!
AP_Math: add sqrt_controller function
AC_AttitudeControl: use sqrt_controller function
AC_PosControl: Use sqrt_controller function
AC_AttitudeControl: cleanup angle_ef_roll_pitch_rate_ef_yaw_smooth