board-status: avoid shell error

[ $3 -eq 1 ] fails if no third argument is given.
[ "$3" -eq 1 ] still fails.

Doing a string comparison is robust across shells.

Change-Id: I3ee388fdbe51b7ab9344d86e67827654714d3191
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6576
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>