HAL_AVR: run MPU6k and MS5611 at 4MHz not 500kHz
AP_Baro: check baro at exactly 100Hz

prevent waits for samples
Copter: give UARTA 512 bytes

this reduces load by allowing more contiguous writes to the serial
buffers
HAL_AVR: added specialised transfer functions for MPU6k and APM1 ADC

this saves quite a lot of time in interrupts
HAL_AVR: Improved AVRTimer micros() and millis()

- More efficient code by using 16-bit timer
- micros() now has proper 1 us resolution and less overhead
- millis() has less overhead
- removed unneeded/unwanted initializatin of timers in AVRTimer::init()