waf: add option --no-submodule-update

That is useful when the user wants to build with one or more submodules at a
specific revision. In case of a developer, that is useful for when a submodule
is in modification process and she needs to (constantly) test the build.

wscript

@@ -54,6 +54,13 @@ def options(opt):
                  action='store_true',
                  help='Output all test programs')
 
+    g.add_option('--no-submodule-update',
+                 dest='submodule_update',
+                 action='store_false',
+                 default=True,
+                 help='Don\'t update git submodules. Useful for building ' +
+                      'with submodules at specific revisions.')
+
 def configure(cfg):
     cfg.env.BOARD = cfg.options.board
     # use a different variant for each board
@@ -97,6 +104,9 @@ def configure(cfg):
         'SKETCHBOOK="' + cfg.srcnode.abspath() + '"',
     ])
 
+    if cfg.options.submodule_update:
+        cfg.env.SUBMODULE_UPDATE = [True]
+
 def collect_dirs_to_recurse(bld, globs, **kw):
     dirs = []
     globs = Utils.to_list(globs)
@@ -198,6 +208,7 @@ def build(bld):
 
     _build_cmd_tweaks(bld)
 
+    if bld.env.SUBMODULE_UPDATE:
         bld.add_group('git_submodules')
         for name in bld.env.GIT_SUBMODULES:
             bld.git_submodule(name)