MAKEALL: Add options for incremental building

--continue will allow you to <ctrl-c> the MAKEALL and pick up where
you left off.

--rebuild-errors will allow you to rebuild only those boards which
had trouble on the last run of MAKEALL, allowing you to quickly test
a simple fix on just those boards.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>