waf: use ardupilotwaf as a Waf tool

That will make it possible to replace calls of the form
`ardupilotwaf.<method>(bld, <keyword-args...>)` with
`bld.<method>(<keyword-args...>)` in the wscripts.

Advantages of that approach:
 - there is no need to import ardupilotwaf in every single wscript
 - it follows the same standard used by c and cxx tools (like bld.program,
   bld.stlib etc)
 - semantically, ap_program, ap_stlib, example etc are all build related
   methods, so it makes sense to bind them to the build context
 - from the wscripts' perspective, the code is cleaner, since ardupilotwaf,
   which is not specific to just build contexts, isn't *explictly* used

wscript

@@ -117,6 +117,7 @@ def list_boards(ctx):
     print(*boards.get_boards_names())
 
 def build(bld):
+    bld.load('ardupilotwaf')
     bld.load('gtest')
 
     #generate mavlink headers
@@ -137,11 +138,10 @@ def build(bld):
     # the tools and examples. This is the first step until the
     # dependency on the vehicles is reduced. Later we may consider
     # split into smaller pieces with well defined boundaries.
-    ardupilotwaf.ap_stlib(
-        bld,
+    bld.ap_stlib(
         name='ap',
         vehicle='UNKNOWN',
-        libraries=ardupilotwaf.get_all_libraries(bld),
+        libraries=bld.get_all_libraries(),
         use='mavlink',
     )
     # TODO: Currently each vehicle also generate its own copy of the