waf: allow partial builds with new mavlink as submodule

The task generator for the mavlink headers is given a name 'mavlink' and is
added as a dependency to all local static libraries, because all of them use
sources that include mavlink headers. It would be nice to have a good way to
detect sources that use mavlink headers or to declare list of libraries that
use the headers, so that the dependency could be added automatically.

Additionally, there's a minor change in syntax for the task generator: using
strings directly instead of lists.

APMrover2/wscript

@@ -24,6 +24,7 @@ def build(bld):
             'AP_ServoRelayEvents',
             'PID',
         ],
+        use='mavlink',
     )
 
     ardupilotwaf.program(

AntennaTracker/wscript

@@ -12,6 +12,7 @@ def build(bld):
         libraries=ardupilotwaf.COMMON_VEHICLE_DEPENDENT_LIBRARIES + [
             'PID',
         ],
+        use='mavlink',
     )
 
     ardupilotwaf.program(

ArduCopter/wscript

@@ -33,6 +33,7 @@ def build(bld):
             'AP_Relay',
             'AP_ServoRelayEvents',
         ],
+        use='mavlink',
     )
 
     ardupilotwaf.program(

ArduPlane/wscript

@@ -34,6 +34,7 @@ def build(bld):
             'AP_Motors',
             'AC_PID'
         ],
+        use='mavlink',
     )
 
     ardupilotwaf.program(

wscript

@@ -120,9 +120,12 @@ def list_boards(ctx):
 def build(bld):
 
     #generate mavlink headers
-    bld(features=['mavgen'],
-        source=['modules/mavlink/message_definitions/v1.0/ardupilotmega.xml'],
-        target=['libraries/GCS_MAVLink/include/mavlink/v1.0/'])
+    bld(
+        features='mavgen',
+        source='modules/mavlink/message_definitions/v1.0/ardupilotmega.xml',
+        target='libraries/GCS_MAVLink/include/mavlink/v1.0/',
+        name='mavlink',
+    )
 
     # NOTE: Static library with vehicle set to UNKNOWN, shared by all
     # the tools and examples. This is the first step until the
@@ -133,6 +136,7 @@ def build(bld):
         name='ap',
         vehicle='UNKNOWN',
         libraries=ardupilotwaf.get_all_libraries(bld),
+        use='mavlink',
     )
     # TODO: Currently each vehicle also generate its own copy of the
     # libraries. Fix this, or at least reduce the amount of