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>
diff --git a/tools/buildman/README b/tools/buildman/README
index 734ada6..f63f278 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -629,6 +629,28 @@
 increases, and vice versa.
 
 
+Providing 'make' flags
+======================
+
+U-Boot's build system supports a few flags (such as BUILD_TAG) which affect
+the build product. These flags can be specified in the buildman settings
+file. They can also be useful when building U-Boot against other open source
+software.
+
+[make-flags]
+at91-boards=ENABLE_AT91_TEST=1
+snapper9260=${at91-boards} BUILD_TAG=442
+snapper9g45=${at91-boards} BUILD_TAG=443
+
+This will use 'make ENABLE_AT91_TEST=1 BUILD_TAG=442' for snapper9260
+and 'make ENABLE_AT91_TEST=1 BUILD_TAG=442' for snapper9g45. A special
+variable ${target} is available to access the target name (snapper9260 and
+snapper9g20 in this case). Variables are resolved recursively.
+
+It is expected that any variables added are dealt with in U-Boot's
+config.mk file and documented in the README.
+
+
 Other options
 =============