kernel: mtdsplit_squashfs: Align with the erase-block size

kernel: mtdsplit_squashfs: Align with the erase-block size

On most image types the rootfs ends at an erase-block. However, at least
with brnImages this is not the case: while the partitions are aligned
with the erase-block size there is a 12 byte footer at the end of the
partition which must not be touched by any filesystem. This lead to a
rootfs_data partition which was not aligned properly (and thus ended up
being readonly):
	0x000000480000-0x00000085a800 : "rootfs_data" (128 KiB EB)

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48263 3c298f89-4303-0410-b956-a3cf2f4a3e73