firmware-utils/mktplinkfw: allow to reserve space in the generated images

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39396 3c298f89-4303-0410-b956-a3cf2f4a3e73
ar71xx: image: reserve 4 erase blocks on the TP-Link devices

This ensures that the generated images will have enough erase
blocks for JFFS2.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39397 3c298f89-4303-0410-b956-a3cf2f4a3e73