Add an interrupt-safe way of comparing a timestamp with the current time.
Add an interrupt-safe way of storing the current time into a timestamp.
Use hrt_elapsed_time() in cases where we can't be sure the timestamp won't change under us.
Merge branch 'px4io-i2c-memory-squeeze' of https://github.com/PX4/Firmware into px4io-i2c-memory-squeeze