Add MSP capability bit so clients can detect Cleanflight and use an
appropriate API.  Renumber some cleanflight specific MSP commands,
Cleanflight will use the range 32-63 for new MSP commands.
Add MPU6500 SPI driver.  Move sensor initialisation defines into target
specific headers.
Initial stab at supporting MPU6500.
Make I2C driver fail gracefully when I2C hardware hasn't been initialized (on boards w/o I2C sensors)
Slight cleanup of MPU6050 driver to use same constants as MPU6500. Both drivers should probably get merged, the operation is nearly identical with only bus access being different.
Fix initialization period for MPU6500, would cause strange effects if not followed.
Correcting MPU6500 driver multibyte reads - deasserting CS after read register write was a bad idea.