buildman: Permit branch names with an embedded '/'

buildman: Permit branch names with an embedded '/'

At present buildman naively uses the branch name as part of its directory
path, which causes problems if the name has an embedded '/'.

Replace these with '_' to fix the problem.

Reported-by: Steve Rae <srae@broadcom.com>
Signed-off-by: Simon Glass <sjg@chromium.org>