scripts/config: fix an option processing error that caused duplication in...

scripts/config: fix an option processing error that caused duplication in diffconfig.sh output (#13384)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36522 3c298f89-4303-0410-b956-a3cf2f4a3e73