base-files: add back missing function for find_mtd_part to /lib/functions.sh (fixes #15496)

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40426 3c298f89-4303-0410-b956-a3cf2f4a3e73