Improvements to the HX stream protocol implementation.
Add the ability to cancel a begin/end perf counter if the begin turns out to have been in error.
Fix an issue with the pwm_servo driver when using one of the STM32 advanced timers.
Add direct-access methods to the base Device class, so that there's a common way of talking to drivers regardless of which of the specialised classes they derive from.

Make the Device destructor public and virtual, so that arbitrary devices can be deleted. Likewise for classes that derive from it.

Make Device::init public so that arbitrary devices can be initialised after being returned by factories.
Add size and flush methods to the ringbuffer class.
Make it possible to create a cdev without automatically creating a device node.
Add a top-level Makefile rule for building "everything" as a test.
Fixed code style for meas_airspeed.cpp
Hotfix: MEAS Airspeed sensor fixes from Sarthak Kaingade
Merge branch 'master' of github.com:PX4/Firmware into mpu6k_queue
Equipped MPU6k driver with Butterworth for accel and gyro
Increased MPU6K poll and sampling rate to 1 KHz