Avoid needing to know about the internal .bm-work directory, by passing
the -w flag to buildman.

This is not needed on travis since the -w flag is already used (from a
previous patch).

Drop the -P flag since this has no effect if -w is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>