optionrom: fix bugs in signrom.sh

signrom.sh has multiple bugs:

- the last byte is considered when calculating the existing checksum, but not
  when computing the correction
- apprently the 'expr' expression overflows and produces incorrect results with
  larger roms
- if the checksum happened to be zero, we calculated the correction byte to be
  256

Instead of rewriting this in half a line of python, this patch fixes the bugs.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

pc-bios/optionrom/signrom.sh

diff --git a/pc-bios/optionrom/signrom.sh b/pc-bios/optionrom/signrom.sh
index 975b27d..9dc5c63 100755
--- a/pc-bios/optionrom/signrom.sh
+++ b/pc-bios/optionrom/signrom.sh
@@ -31,14 +31,13 @@
 size=$(( $x * 512 - 1 ))
 
 # now get the checksum
-nums=`od -A n -t u1 -v "$1"`
+nums=`od -A n -t u1 -v -N $size "$1"`
 for i in ${nums}; do
     # add each byte's value to sum
-    sum=`expr $sum + $i`
+    sum=`expr \( $sum + $i \) % 256`
 done
 
-sum=$(( $sum % 256 ))
-sum=$(( 256 - $sum ))
+sum=$(( (256 - $sum) % 256 ))
 sum_octal=$( printf "%o" $sum )
 
 # and write the output file