buildman: Don't default to -e when using -s

When using summary mode (-s) we don't always want to display errors.
Allow this option to be omitted.

Series-to: u-boot
Series-cc: albert
Change-Id: I6b37754d55eb920ecae114fceba55834b43ea3b9
Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>