ar71xx: image: fix size verification in the CatFiles macro

The macro checks the size of the rootfs image even if
the second parameter of the macro contains the size of
an eraseblock. It is wrong because the size of the
kernel image will be adjusted to the next eraseblock
and the resulting image might be too large.

Fix the macro to check the size of the final image
instead of the rootfs image.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39383 3c298f89-4303-0410-b956-a3cf2f4a3e73