pygrub: add Debian extlinux.conf path

This is Debian bug #697407.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=697407

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit 258d27a1d9)

tools/pygrub/src/pygrub

@@ -430,6 +430,7 @@ class Grub:
                             "/boot/grub2/grub.cfg", "/grub2/grub.cfg"]) + \
                        map(lambda x: (x,grub.ExtLinuxConf.ExtLinuxConfigFile),
                            ["/boot/isolinux/isolinux.cfg",
+                            "/boot/extlinux/extlinux.conf",
                             "/boot/extlinux.conf"]) + \
                        map(lambda x: (x,grub.GrubConf.GrubConfigFile),
                            ["/boot/grub/menu.lst", "/boot/grub/grub.conf",