autotest: add easier HIL start scripts for rover/plane
HAL_AVR: avoid writing bytes to EEPROM that are already correct

this speeds up writing of waypoint data, reducing latency
Rover: tweaks to new scheduler code
Plane: switch to new AP_Scheduler

this gives us more accurate task scheduling in ArduPlane, plus better
monitoring of task timing (via SCHED_DEBUG)
Plane: update_commands() can take 7ms if it needs to write a waypoint