Revert "efi: Allow easy selection of serial-only operation"

This commit re-introduced a migrated CONFIG symbol to the board header
file.  These changes should likely be handled via documentation instead,
as well.

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