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!
Frame_params: Updated 3DR_X8-M_RTF.param
HAL_Linux: added low level DSM decoder

based on dsm.c from PX4 project
HAL_Linux: enable DSM power pin
HAL_Linux: added DSM/Spektrum RCInput support

this decodes DSM using the RCIN pulses from the PRU
Linux_HAL_Essentials: P8.15 is an input pin
Copter: log CURR message at 10hz
Copter: add throttle_zero state
Copter: Change all zero throttle checks that should be conservative to use ap.throttle_zero
Copter: throttle zero debounce to separate function

Also initialise throttle_zero  to true on startup
Treat throttle less than zero as zero
Rover: fixed skid steering

the main issue was the use of the last throttle in the throttle slew
rate control, but manual skid steering was also broken
HAL_Linux: don't accept less than 5 input channels
Rover: prepare 2.47beta1
AP_RangeFinder: handle all I2C rangefinder types on PX4 in PX4Firmware
INS: add gyro_calibrated_ok_all method

This returns true if the gyros have been calibrated successfully