mavparse.py: append ENUM_END only after merging/sorting

With custom definition files, this way we can add new enum values to the end of existing (common) enums and the ENUM_END is calculated and placed correctly.

Eg. I wanted to do something like this in our autoquad.xml file:

		<!-- extend MAV_DATA_STREAM -->
		<enum name="MAV_DATA_STREAM">
			<entry value="13" name="MAV_DATA_STREAM_PROPULSION">
				<description>Motor/ESC telemetry data.</description>
			</entry>
		</enum>

The way it is now, the MAV_DATA_STREAM_ENUM_END value would = 13, and I would get a duplicate value error unless I used 14 or higher for my new entry... but then the ENUM_END was wrong, defeating the purpose.

Thanks,
-Max