HAL_PX4: do not overwrite disabled channels with zeros

Fixes #1321
HAL_PX4: work-around for periodic enable_ch() calls
HAL_PX4: minor tidy up of if statement
AP_HAL_PX4 : make UARTDriver capable to be called from different threads.

The modification allows the read and write functions to be called by any thread but the calling thread must be the last one that called the begin() function.
AP_Mount: Axis mask speedup

Using a simple bit mask to avoid calculating an unneeded ATAN2() for AVR users.