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.