[buildroot] image.mk: don't squash file permissions in JFFS2 images

[buildroot] image.mk: don't squash file permissions in JFFS2 images

Currently we call mkfs.jffs2 --squash, which will change
both file permissions and owners.

If we have some file with suid bits, it will clear these bits.

Thus it's enough to just squash file owners, by replacing that
argument to --squash-uids.

Signed-off-by: Jonh Wendell <jonh.wendell@oiwifi.com.br>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34397 3c298f89-4303-0410-b956-a3cf2f4a3e73