mpu6000: Support for up to three accels / gyros
L3GD20: Support for up to three gyros
uORB: Support up to three topics per sensor
sdlog2: Add support for up to three IMU sensors
drivers: Up to three units support
Build fixes for example
sensors: Support for up to three sensors of the IMU types
LSM303D: Support for tertiary sensors
HMC5883 driver: stack size adjustments of start handler and start return review / comments
L3GD20 driver: stack size adjustments of start handler and start return review / comments
LSM303D: start handler fix and start routine review
airspeed drivers: Start handlers stack fixes and start handler review
MPU6K: Start handler startup and stack review and adjustments
Merge pull request #1187 from PX4/mavlink_strict

Mavlink strict
Merge branch 'master' of github.com:PX4/Firmware into sensor_startup_cleanup
Making lowpass filter init bullet proof
device driver: Fix compile warnings
LSM303D: Ensure init and constructors
L3GD20: Ensure init and constructors
HMC driver: Full initialization