buildman: Keep more outputs with the --keep-outputs flag

buildman: Keep more outputs with the --keep-outputs flag

When told to keep outputs, be much more liberal in what files we keep.
In addition to adding 'MLO', keep anything that matches u-boot-spl.* (so
that we keep the map file as well) and anything we generate about
'u-boot itself.  A large number of bootable formats now match this and
thus it's easier to build many targets and then boot them afterwards
using buildman.

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