buildman: Print a message when removing old directories

When buildman starts, it prepares its output directory by removing any old
build directories which will not be used this time. This can happen if a
previous build left directories around for commit hashes which are no-longer
part of the branch.

This can take quite a while, so print a message to indicate what is going
on.

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