USB fixes from Petteri Aimonen, cherry-picked.
Maxbotix I2C Sonar Support
Changes from pull request feedback
Missed the accel reference here
Fix white space
write adc values to sensors combined and log them
scale the saved adc values to get voltage
Added minimum set of IO MAVLink text messages, report critical errors such as in-air restarts
Merge pull request #216 from thomasgubler/adc

write adc values to sensors combined and log them
Merge branch 'master' of github.com:PX4/Firmware into px4io-i2c
Merge pull request #214 from ghulands/sonar

Maxbotix I2C Sonar Support
Merge branch 'master' of github.com:PX4/Firmware into px4io-i2c
Add missing mixer ok check in override mode, clear FMU lost alarm when setting FMU_OK flag, print AP RX timeout in production mode as well
Merge branch 'px4io-i2c' of github.com:PX4/Firmware into px4io-i2c
Merge branch 'px4io-i2c' of github.com:PX4/Firmware into px4io-i2c
After the mb12xx driver was merged way too early, make the best out of it and fix up the init phase to the driver bails out if there is no sensor connected
adding missing include
Merge pull request #217 from thomasgubler/px4io-i2c

adding missing include
Merge branch 'px4io-i2c' of github.com:PX4/Firmware into px4io-i2c
Refactored debug level into proper register, px4io status now correctly reads it. Added more of the missing alarms clear logic, alarms reporting now consistent. Adding missing sign change on mode switch, fixes override issue when attempting to switch to auto mode. Pending outdoor tests