buildman: Allow make flags to be specified for each board

There are a few make options such as BUILD_TAG which can be provided when
building U-Boot. Provide a way for buildman to pass these flags to make
also.

The flags should be in a [make-flags] section and arranged by target name
(the 'target' column in boards.cfg. See the README for more details.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builder.py

@@ -253,6 +253,7 @@ class BuilderThread(threading.Thread):
                     args.extend(['-j', str(self.builder.num_jobs)])
                 config_args = ['%s_config' % brd.target]
                 config_out = ''
+                args.extend(self.builder.toolchains.GetMakeArguments(brd))
 
                 # If we need to reconfigure, do that now
                 if do_config: