waf: mavgen: set system env during build instead of configure

By setting env.env during configure time makes changes be for the whole build
system scope and persistently, which may cause problems for other tasks and
tools.

Tools/ardupilotwaf/mavgen.py

@@ -37,7 +37,11 @@ def process_mavgen(self):
         outputs.append(t)
 
     self.source = []
-    self.create_task('mavgen', inputs, outputs)
+
+    task = self.create_task('mavgen', inputs, outputs)
+
+    task.env.env = dict(os.environ)
+    task.env.env['PYTHONPATH'] = task.env.MAVLINK_DIR
 
 def configure(cfg):
     """
@@ -47,7 +51,6 @@ def configure(cfg):
     cfg.check_python_version(minver=(2,7,0))
 
     env = cfg.env
-    cfg.env.env = dict(os.environ)
 
     cfg.start_msg('Checking for message_definitions')
     if not cfg.srcnode.find_resource('modules/mavlink/message_definitions/v1.0/ardupilotmega.xml'):
@@ -60,6 +63,5 @@ def configure(cfg):
     env.MAVLINK_HEADERS = cfg.bldnode.make_node('/libraries/GCS_MAVLink/include/mavlink/v1.0/').abspath()
 
     env.MAVGEN = env.MAVLINK_DIR  + '/pymavlink/tools/mavgen.py'
-    cfg.env.env['PYTHONPATH'] = env.MAVLINK_DIR
 
     env.MAV_MSG_DEFS = cfg.srcnode.find_resource('modules/mavlink/message_definitions/v1.0/ardupilotmega.xml').abspath()