buildman: Allow building within a subdir of the current dir

This is useful in some situations, in particular with -w and when building
in-tree. Now that we are more careful about what we remove in
_PrepareOutputSpace(), it should be safe to relax this restriction.

Update the progress information also so it is clear what buildman is
doing. Remove files can take a long time.

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