waf: add --define command-line option

Allows CPP defines to be specified on command-line

Eases enable/disable of optional features

Tools/ardupilotwaf/ardupilotwaf.py

@@ -563,6 +563,10 @@ arducopter and upload it to my board".
         action='store_true',
         help='Output all test programs.')
 
+    g.add_option('--define',
+        action='append',
+        help='Add C++ define to build.')
+
     g = opt.ap_groups['clean']
 
     g.add_option('--clean-all-sigs',

Tools/ardupilotwaf/boards.py

@@ -136,6 +136,13 @@ class Board:
         # make easy to override them. Convert back to list before consumption.
         env.DEFINES = {}
 
+        # potentially set extra defines from an environment variable:
+        if cfg.options.define is not None:
+            for (n, v) in [d.split("=") for d in cfg.options.define]:
+                cfg.msg("Defining: %s" % (n, ), v)
+                env.CFLAGS += ['-D%s=%s' % (n, v)]
+                env.CXXFLAGS += ['-D%s=%s' % (n, v)]
+
         env.CFLAGS += [
             '-ffunction-sections',
             '-fdata-sections',