this ensures we never block while writing a MAVLink message to a
serial port, by checking the number of available bytes in the serial
transmit buffer and deferring any message that would cause a blocking
write.
This should prevent the main loop from clagging up due to excessive
telemetry data being sent
git-svn-id: https://arducopter.googlecode.com/svn/trunk@3251 f9c3cf11-9bcb-44bc-f272-b75c42450872
