base-files: add mtd_get_part_size helper

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37475 3c298f89-4303-0410-b956-a3cf2f4a3e73
ar71xx: use mtd_get_part_size helper

Drop the local get_mtd_part_size implementation and use
the equivalent function provided by lib/functions.sh.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37476 3c298f89-4303-0410-b956-a3cf2f4a3e73