Merge branch 'arm_safe_fix' into seatbelt_multirotor
Merge branch 'master' into seatbelt_multirotor
position_estimator_inav init bugs fixed
Merge pull request #7 from skelly/l3gd20h_support

Enabled MS5611, nsh on USB by default.
Rework the FMU<->IO connection to use a simple fixed-size DMA packet; this should let us reduce overall latency and bump the bitrate up.

Will still require some tuning.
USB console isn't working. Go back to UART8 which is.
Fix argument parsing in the rgbled command.
Add a test hook for the PX4IO interface. Wire up some simple tests for the serial interface.

Signal quality looks good at 1.5Mbps. Transmit timing is ~450µs per packet, ~20µs packet-to-packet delay spinning in a loop transmitting.
Add DMA error handling.
Add serial error handling.
Add short-packet-reception handling (this may allow us to send/receive shorter packets… needs testing).
Add PX4IOv2 support to the uploader.
Fix printing of PC_COUNT perf counters
iov2 pin definition cleanup sweep
Switch to the 'normal' way of doing register accessors.
Be more aggressive en/disabling DMA in the UART, since ST say you should.
More test work on the px4io side of the serial interface.
One more piece of paranoia when resetting DMA
Reset the PX4IO rx DMA if we haven't seen any traffic in a while; this gets us back into sync.
Use the NuttX built-in crc32, it works fine.
Always send and expect a reply for every message.
Encode the packet type and result in the unused high bits of the word count.
Add an 8-bit CRC to each transmitted packet.