AP_Scheduler: added current_task static

will be used to debug stuck tasks on PX4
HAL_PX4: print overtime message on stuck task

this should make it easier to narrow down stuck task bugs
GPS: Include 75-class CPUs in RTK support since they will have 64 bit floating point values.
GPS: Fix dataflash logging bug in SBP driver
AP_HAL_VRBRAIN: removed empty lines
PX4: disable PX4IO RC handling in a clean way

Upstream PX4 Firmware provides a clean way to disable PX4IO handling by supplying the argument "norc" to "px4io start". After applying this fix to the rc.APM startup script, the quick hack contained in commit 180cceee of diydrones/PX4Firmware can be safely reverted.
PX4: added conversion lib for rotation
PX4: try to startup FMUv3 with right rotations
ph2 support: Fix typo in startup rc
PX4: fixed l3gd20 rotation

based on first flight log
PX4: added comments on FMU3 rotations
AP_Compass: support 3 mags on PX4
Copter: log up to 3 mags
PX4: try to start both hmc5883 sensors

internal + external
Plane: support 3 magnetometers
Rover: support 3 magnetometers
PX4: fixed orientation of internal hmc5883 on FMUv3
GCS_MAVLink: add CMD_NAV_GUIDED to ardupilotmega.xml
GCS_MAVLink: generate after adding CMD_NAV_GUIDED
AP_Mission: add support for MAV_CMD_NAV_GUIDED