Sub: 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.

ArduSub/wscript

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