scripts/ubinize-image.sh: fix buildbot breakage

scripts/ubinize-image.sh: fix buildbot breakage

New Docker based buildslaves install just bare minimum of packages, thus
not having bsdmainutils package installed which provides `hexdump`
utility, leading to the following build breakage on buildbots:

 ubinize-image.sh: 12: /builder/scripts/ubinize-image.sh: hexdump: not found

So this patch simply replaces `hexdump` with `od` utility provided by
coreutils package, which should be likely available.

Co-authored-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>