waf: prepare for using git_submodule tool

We define three build groups and set post_mode as POST_LAZY, which makes one
group be posted only after the previous one. That enables us to correctly
define dynamic sources with submodules and generators that may or may not
depend on submodules (e.g. mavgen - this one depends on mavlink submodule).

The groups are defined below, in that order:
 1. git_submodules: contains taskgens that initialize and update submodules.
 2. dynamic_sources: contains taskgens that generate sources used by the build.
 3. build: contains the normal taskgens.

wscript

@@ -80,6 +80,7 @@ def configure(cfg):
     cfg.load('clang_compilation_database')
     cfg.load('waf_unit_test')
     cfg.load('mavgen')
+    cfg.load('git_submodule')
     cfg.load('gbenchmark')
     cfg.load('gtest')
     cfg.load('static_linking')
@@ -128,7 +129,7 @@ def _build_cmd_tweaks(bld):
             bld.fatal('check: gtest library is required')
         bld.add_post_fun(ardupilotwaf.test_summary)
 
-def _create_common_taskgens(bld):
+def _build_dynamic_sources(bld):
     bld(
         features='mavgen',
         source='modules/mavlink/message_definitions/v1.0/ardupilotmega.xml',
@@ -142,6 +143,7 @@ def _create_common_taskgens(bld):
         ],
     )
 
+def _build_common_taskgens(bld):
     # 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
@@ -198,11 +200,23 @@ def _build_recursion(bld):
         bld.recurse(d)
 
 def build(bld):
+    bld.post_mode = Build.POST_LAZY
+
     bld.load('ardupilotwaf')
     bld.load('gtest')
 
     _build_cmd_tweaks(bld)
-    _create_common_taskgens(bld)
+
+    bld.add_group('git_submodules')
+    for name in bld.env.GIT_SUBMODULES:
+        bld.git_submodule(name)
+
+    bld.add_group('dynamic_sources')
+    _build_dynamic_sources(bld)
+
+    bld.add_group('build')
+    _build_common_taskgens(bld)
+
     _build_recursion(bld)
 
 ardupilotwaf.build_command('check',