Navio: RCOutput fix, RCOutput optimization, RGB LED support, LinuxSPIUARTDriver for U-blox, AK8363_MPU9250 compass support

External clocking for PCA9685 solves the issue with PWM values drifting.
I2C bandwidth required for PCA9685 is reduced by 40% by writing only falling edge bytes.
Added support for RGB LED on Navio, in order to avoid copypasting PX4 code parts of AP_Notify have been rewritten.
Added LinuxSPIUARTDriver which allows to work with U-blox over SPI. Currently, using internal GPS does not allow to use external GPS on Navio, but the solution for that is on it's way.
Added support of MPU9250 internal compass - AK8963.