waf: mavgen: ensure order without bld.add_group()

This is a better approach, because there's no need of calling add_group() for
each time a mavgen task generator is created.

Tools/ardupilotwaf/mavgen.py

@@ -14,6 +14,7 @@ class mavgen(Task.Task):
     """generate mavlink header files"""
     color   = 'GREEN'
     run_str = '${PYTHON} ${MAVGEN} --lang=C --wire-protocol=1.0 --output ${TGT} ${SRC}'
+    before  = 'cxx c'
 
     def post_run(self):
         super(mavgen, self).post_run()

wscript

@@ -124,7 +124,6 @@ def build(bld):
         source=['modules/mavlink/message_definitions/v1.0/ardupilotmega.xml'],
         target=['libraries/GCS_MAVLink/include/mavlink/v1.0/'])
 
-    bld.add_group()
     # NOTE: Static library with vehicle set to UNKNOWN, shared by all
     # the tools and examples. This is the first step until the
     # dependency on the vehicles is reduced. Later we may consider