Fix checksum writing in signboot.sh

The printf command takes an octal value after \, so we have to convert
our decimal representation to octal first and then write it.

This unbreaks extboot signing. Multiboot wasn't affected yet because
the checksum was < 8.

Spotted and first patch by Glauber Costa <glommer@redhat.com>.
Printf idea by Paolo Bonzini <bonzini@gnu.org>.

Signed-off-by: Alexander Graf <agraf@suse.de>
CC: Glauber Costa <glommer@redhat.com>
CC: Paolo Bonzini <bonzini@gnu.org>
CC: Jan Ondrej <ondrejj@salstar.sk>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
