MAVLink XML change to allow exponentiation operator in bitmask flag

[hamishwillee]

FYI We're modifying the MAVLink XSD validation file in ArduPilot/pymavlink#920 to allow a bitmask value to be declared using the Python exponentiation operator.

This makes it easier to immediately visualize which bit is set by a flag, because you can use the syntax

<entry value="2**15" name="BIT15" />

rather than

<entry value="32768" name=" BIT15" />

This is transparent to mavgen because the parser evaluates the number before creating the generated headers. However it may affect other parsers.
We're not updating the XML yet to allow other parsers time to update.

Any concerns, please raise on ArduPilot/pymavlink#920