AltitudeHold: tune the amount of tilt compensation

The existing code would still drop during forward flight,
because of the non-linearities of open loop ESCs. This
allows tuning the amount of tilt compensation.
AltitudeHold: make compensation amount a uint16

Being able to only go to the power 2.5 was insufficient to
maintain forward flight at steeper angles.
UAVOFrSKYSPortBridge: UAVOs extended
UAVOFrSKYSPortBridge: added module source skeleton
UAVOFrSKYSPortBridge: module enable option in GCS
PiOS: STM32F3 USART driver enhanced with single-wire half-duplex mode

This is used for FrSky S.PORT bus so telemetry transmitter can be directly
connected to the TX pin of USART without any additional external circuitry
on STM32F3 targets.
UAVOFrSKYSPortBridge: first functional code and support for flyingf3 target
UAVOFrSKYSPortBridge: module name "UAVOFrSkySPortBridge" changed to "UAVOFrSKYSPortBridger"

To be consistent with existing UAVOFrSKYSensorHubBridge module
UAVOFrSKYSPortBridge: added to sparky target
UAVOFrSKYSPortBridge: added to flyingf4 target
UAVOFrSKYSPortBridge: added to freedom target
UAVOFrSKYSPortBridge: added to quanton target
UAVOFrSKYSPortBridge: fixed copy&paste error in freedom board file
UAVOFrSKYSPortBridge: added to revolution target
UAVOFrSKYSPortBridge: added to revomini target
UAVOFrSKYSPortBridge: removed testing value from frsky_encode_gps_alt()
UAVOFrSKYSPortBridge: fixed comments and function names
UAVOFrSKYSPortBridge: fixed bug in state FRSKY_STATE_WAIT_TX_DONE

Receiver ignored one more byte than neccessary (poll request byte of
consequent poll) and therefore effectively reduced bandwidth by 50%
UAVOFrSKYSPortBridge: appended more sensor IDs to which we response

It's increasing bandwidth by approx. 20%. Pretty responsive now.
Updated tools.mk for compatibility with Mac OS 10.10