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: NAlexander 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: NAnthony Liguori <aliguori@us.ibm.com>
Message-Id: 

pc-bios/optionrom/signrom.sh

@@ -39,7 +39,8 @@ done
 
 sum=$(( $sum % 256 ))
 sum=$(( 256 - $sum ))
+sum_octal=$( printf "%o" $sum )
 
 # and write the output file
 cp "$1" "$2"
-printf "\\$sum" | dd of="$2" bs=1 count=1 seek=$size conv=notrunc 2>/dev/null
+printf "\\$sum_octal" | dd of="$2" bs=1 count=1 seek=$size conv=notrunc 2>/dev/null