brcm63xx: do not align squashfs rootfs start

We do not need to align the start of read only rootfs's to erase blocks.
This allows us to write the squashfs rootfs directly behind the kernel,
potentially freeing up one erase block.

We still need to align for jffs2, so add a flag for imagetag to
optionally align the rootfs start.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 44556