HAL_Linux: use CLOCK_MONOTONIC_RAW to ensure clock never goes backwards

this avoids ntp mucking with APM timing
AP_HAL_FLYMAPLE: Improved libmaple version detection
AP_HAL_FLYMAPLE: UARTDriver begin now honours txS == 0 which means no change
AP_HAL_FLYMAPLE: FlymaplePortingNotes.txt improvements to install/build
documentation.
AP_HAL_FLYMAPLE: UARTDriver now uses the new libmaple tx ring buffers

This requires the libmaple fork at
https://github.com/mikemccauley/libmaple.git
which includes low level support for buffered usart transmits