buildman: Allow building of current source tree

Originally buildman had some support for building the current source tree.
However this was dropped before it was submitted, as part of the effort to
make it faster when building entire branches.

Reinstate this support. If no -b option is given, buildman will build the
current source tree.

Reported-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>