HAL_PX4: prevent excessive writes on startup from blocking

this could cause copter on PX4 to hang on startup
Copter: fixed baro-only preflight cal

this was causing a full level, which mucked up the INS calibration
HAL_AVR: switch back to 500kHz SPI for MPU6k until we understand the issues

this is for new plane release. We need for analysis before we can
re-enable high speed SPI
Plane: improved perf monitoring with SCHED_DEBUG
Rover: fixed performance monitoring

now the same as plane
Plane: ready for 2.75 release
Copter: fixed HIL build