buildman: Add verbose option to display errors as they happen

Normally buildman operates in two passes - one to do the build and another
to summarise the errors. Add a verbose option (-v) to display build problems
as they happen. With -e also given, this will display errors too.

When building the current source tree (rather than a list of commits in a
branch), both -v and -e are enabled automatically.

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