ar71xx: fix mtdpartsize macro in image Makefile (#14961)

The mtdpartsize macro triggers shell errors on various Linux distributions
when the partname argument $(1) does not appear within the partmap $(2).

Change the sed pattern to only emit anything if a successful substitution
occured and only evaluate the arithmetic expression if something was printed
by the sed program.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39583 3c298f89-4303-0410-b956-a3cf2f4a3e73