buildman: Support in-tree builds

At present buildman always builds out-of-tree, that is it uses a separate
output directory from the source directory. Normally this is what you want,
but it is important that in-tree builds work also. Some Makefile changes may
break this.

Add a -i option to tell buildman to use in-tree builds, so that it is easy
to test this feature.

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