Merge branch 'px4io-i2c-memory-squeeze' of https://github.com/PX4/Firmware into px4io-i2c-memory-squeeze

Add an interrupt-safe way of storing the current time into a timestamp.

Tested by Felipe Reis <felipereis87@gmail.com>

Added additional debug / uORB example

Switched to debug statement which is more efficient regarding stack usage, only printing at debug level 2 or higher.

poll() is not interrupted by signals, whereas usleep() is

this leaves us enough flash to fit APM

needed for APM startup

the autopilot code needs to know that in-air restart may happen, so it
should be something that is enabled, rather than on by default.

this is useful for startup scripts testing for auto-upgrade of add-on 
board firmware

these are far too large (777 kbyte) and we can't fit them with the
ArduCopter flight code

this matches the PX4IO interface

this allows for PPM input with no IO board

Nuttx merge 5596

Finished and tested in-air restore of arming state, as long as both boards reset at the same time armings state is now retained