The NOR flash rootfs images stored in a sysupgrade.tar must end with the
JFFS2 marker. Otherwise, devices like OpenMesh A42/A62 are not able to
calculate the md5sum of the fixed squashfs part and store it inside the
u-boot-env.

But the commit ee76bd11 ("images: fix boot failures on NAND with small
sub pages") adds up to 1020 0x00 bytes after the 0xdead0de EOF marker. The
calculated md5sum will be wrong due do this change and u-boot will fail to
boot the newly flashed device with a message like:

  Validating MD5Sum of 'vmlinux'...
  Passed!
  Validating MD5Sum of 'rootfs'...
  Failed!
      583a1b7b54b8601efa64ade42742459b != 8850ee812dfd7638e94083329d5d2781

  Data validation failed!

and boot the old image again.

Since the original change should not change the behavior of NOR images,
just check for the deadc0de marker at the end of the squashfs-jffs2 image
do avoid the problematic behavior for these images.

Fixes: ee76bd11 ("images: fix boot failures on NAND with small sub pages")
Signed-off-by: Sven Eckelmann <sven@narfation.org>