SquashFS has a minimum block size of at least 1k, so we need to make
sure the last data block is also at least that big.

This is not an issue on NOR or SD CARD devices, since their rootfs
partitions go all the way to the end of the usable space.

But on NAND with ubiblock, the rootfs partition will be the exact space,
rounded up to LEB size. Unfortunately, some NAND chips with small sub
pages have a LEB size of x.5 kiB. This can cause the the last data block
to be less than 1k, which will cause the last block to be inaccessible,
causing boot failures as seen on MR24:

[    1.532960] block ubiblock0_3: created from ubi0:3(rootfs)
[    1.538457] ubiblock: device ubiblock0_3 (rootfs) set to be root filesystem
[    1.552847] SQUASHFS error: squashfs_read_data failed to read block 0x621472
[    1.559896] squashfs: SQUASHFS error: unable to read id index table
[    1.566474] VFS: Cannot open root device "(null)" or unknown-block(254,0): error -5

Since on most NOR devices, the start of the squashfs partition is not
aligned. Since the start of the rootfs_data partition there is dependend
on the SquashFS size, we cannot just always pad it, as the padding could
creep into the rootfs_data partition, breaking jffs2.

So fix this by ensuring a squashfs rootfs is always a multiple of 1k
only for UBI and NAND sysupgrade images.

Fixes #2460 without affecting NOR devices.

Tested-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>