AP_HAL_SMACCM: Semaphores fixup
AP_HAL: add Scheduler methods in_timerprocess, system_initialize
AP_HAL_AVR: Implement new scheduler methods
AP_HAL_AVR_SITL: Implement new Scheduler methods
AP_HAL_Empty: Stub out new Scheduler methods
AP_InertialSensor_MPU6000: fix sync/async semaphore usage with new scheduler
AP_HAL_SMACCM: Implement new scheduler methods.

- Set the system initialized after call to "setup" in main.
AP_Baro_MS5611: Don't panic if taking semaphore fails during init.

- The MPU6000 holds on to the I2C semaphore for quite some time during
  init, which caused a panic when the MS5611 is also on I2C.