buildman: Don't default to -e when building current source

We probably don't need to enable this option by default. It is useful to
display only failure boards (not errors) and it is easy to add -e if it
is required. Also update the docs.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>