Fix code extracting the xorriso version

The code was assuming that the first line of output was something like
this (and this is the case of the cd builder machine that uses a custom
built xorriso):
> GNU xorriso 1.2.6 : RockRidge filesystem manipulator, libburnia project.

But the output of the version packaged in Debian does not have the GNU
string:
> xorriso 1.3.2 : RockRidge filesystem manipulator, libburnia project.

We fix this issue by extracting the version from the line starting with
"xorriso version" which does not have this discrepancy.

debian/changelog

@@ -7,6 +7,9 @@ debian-cd (3.1.15) UNRELEASED; urgency=low
   [ Samuel Thibault ]
   * Make kfreebsd & hurd images usbkey-bootable. Closes: #739422
 
+  [ Raphaël Hertzog ]
+  * Fix code extracting the xorriso version.
+
  -- Steve McIntyre <93sam@debian.org>  Tue, 11 Feb 2014 01:57:01 +0000
 
 debian-cd (3.1.14) unstable; urgency=medium

tools/boot/jessie/boot-hurd

@@ -23,8 +23,8 @@ fi
 case "$MKISOFS" in
     *xorriso*)
         XORRISO_VER=$($MKISOFS --version 2>&1 | awk '
-            NR==1 {
-                 split($3, ver, ".")
+            /^xorriso version/ {
+                 split($4, ver, ".")
                  print ver[1]*10000+ver[2]*100+ver[3]
             }')
         # OK

tools/boot/jessie/boot-kfreebsd

@@ -30,8 +30,8 @@ fi
 case "$MKISOFS" in
     *xorriso*)
         XORRISO_VER=$($MKISOFS --version 2>&1 | awk '
-            NR==1 {
-                 split($3, ver, ".")
+            /^xorriso version/ {
+                 split($4, ver, ".")
                  print ver[1]*10000+ver[2]*100+ver[3]
             }')
         # OK

tools/boot/jessie/boot-x86

@@ -30,8 +30,8 @@ fi
 case "$MKISOFS" in
     *xorriso*)
         XORRISO_VER=$($MKISOFS --version 2>&1 | awk '
-            NR==1 {
-                 split($3, ver, ".")
+            /^xorriso version/ {
+                 split($4, ver, ".")
                  print ver[1]*10000+ver[2]*100+ver[3]
             }')
         # OK

tools/boot/wheezy/boot-x86

@@ -38,8 +38,8 @@ fi
 case "$MKISOFS" in
     *xorriso*)
         XORRISO_VER=$($MKISOFS --version 2>&1 | awk '
-            NR==1 {
-                 split($3, ver, ".")
+            /^xorriso version/ {
+                 split($4, ver, ".")
                  print ver[1]*10000+ver[2]*100+ver[3]
             }')
         # OK