AP_Scheduler: improved the accuracy of the scheduling code

prevent accumulated micros() errors in run()
autotest: speed up build_all_px4.sh

only build archives once
AP_Scheduler: show slips only for SCHED_DEBUG>1

this allows us to see PERF reports in copter without console writes
slowing things down
HAL_AVR: reduce the latency of semaphore waits

this reduces the average cost of waiting for the MPU6000 semaphore
from the main loop