boot splash: provide back option in Debian release submenu

Useful for people who don't know the <esc> trick :)

From: Michael Prokop <mprokop@sipwise.com>

templates/boot/isolinux/isolinux.cfg

@@ -108,9 +108,15 @@ label prosp2nw
 
 ## begin of Debian menu {{{
 menu separator
+
 menu begin debian-options
 menu title Install specific Debian releases ...
 
+label mainmenu
+menu label ^Back to main menu...
+menu exit
+menu separator
+
 label squeeze
   menu label Install Debian/squeeze 64bit - DHCP
   kernel /boot/%SHORT_NAME%/vmlinuz
@@ -154,7 +160,7 @@ label squeezepuppet
   endtext
 
 label wheezy
-  menu label Install Debian/wheezy 64bit - DHCP
+  menu label Install Debian/wheezy  64bit - DHCP
   kernel /boot/%SHORT_NAME%/vmlinuz
   append initrd=/boot/%SHORT_NAME%/initrd.img live-media-path=/live/%GRML_NAME%/ boot=live bootid=%BOOTID% apm=power-off vga=791 quiet nomce nongcp ssh=sipwise netscript=http://deb.sipwise.com/kantan/deployment.sh debianrelease=wheezy
 
@@ -168,7 +174,7 @@ label wheezy
 
 
 label wheezynw
-  menu label Install Debian/wheezy 64bit - static NW
+  menu label Install Debian/wheezy  64bit - static NW
   kernel /boot/%SHORT_NAME%/vmlinuz
   append initrd=/boot/%SHORT_NAME%/initrd.img live-media-path=/live/%GRML_NAME%/ boot=live bootid=%BOOTID% apm=power-off vga=791 quiet nomce nongcp ssh=sipwise netscript=http://deb.sipwise.com/kantan/deployment.sh ip=192.168.51.123::192.168.51.1:255.255.255.0:debian:eth0:off dns=92.42.136.30 debianrelease=wheezy 
 
@@ -181,7 +187,7 @@ label wheezynw
   endtext
 
 label wheezypuppet
-  menu label Install Debian/wheezy 64bit - Puppet
+  menu label Install Debian/wheezy  64bit - Puppet
   kernel /boot/%SHORT_NAME%/vmlinuz
   append initrd=/boot/%SHORT_NAME%/initrd.img live-media-path=/live/%GRML_NAME%/ boot=live bootid=%BOOTID% apm=power-off vga=791 quiet nomce nongcp ssh=sipwise netscript=http://deb.sipwise.com/kantan/deployment.sh ip=192.168.51.123::192.168.51.1:255.255.255.0:puppet-client42:eth0:off dns=92.42.136.30 puppetenv=production debianrelease=wheezy