Add new safety switch LED blink sequence when both FMU and IO are armed

If both the FMU and the IO board are armed then the secure switch will
blink two times quickly then a pause followed by two quick blinks and
so on.

BlinkM driver

Added delay test to measure comm delays with a led / scope

Made sure IO and FMU obey the lockdown flag when arming motors

#61 px4io spektrum decoder

Cleaned up different uorb topics, cleaned up excessive stack sizes

Fixed missing heading for mtkcustom and nmea

Break out the task stack usage sniffer so we can use it on systems where the task layout has been shrunk (e.g. PX4IO)

Use the right constraint for the output mixer; we might end up wanting more virtual control channels.

Harmonized PPM, S.BUS and DSM input (order: first preference S.Bus, then DSM, then PPM, first available and valid source is chosen), tested with FMU, valid channel inputs

 - Use a separate thread for handing R/C inputs and outputs.
 - Remove all PX4IO R/C receiver configuration; it's all automatic now.
 - Rework the main loop, dedicate it to PX4FMU communications after startup.
 - Fix several issues in the px4io driver that would cause a crash if PX4IO was not responding.

 - enable USART3 for S.bus
 - reduce the scheduling tick to 1ms
 - disable RR scheduling
 - reduce stdio buffer sizes (not used)