Revert "build: remove harmful -nopad option from mksquashfs"

Revert "build: remove harmful -nopad option from mksquashfs"

This reverts commit 1c0290c5cc6258c48b8ba46b4f9c85a21de4f875.

Dropping the nopad can make the padding overflow into the next erase
block on devices using a non-aligned rootfs start. This breaks the jffs2
overlay partition with the following messages:

[   30.343877] jffs2_scan_eraseblock(): End of filesystem marker found at 0x10000
[   30.376512] jffs2: Cowardly refusing to erase blocks on filesystem with no valid JFFS2 nodes
[   30.385253] jffs2: empty_blocks 196, bad_blocks 0, c->nr_blocks 197

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>