sysupgrade: force the user to use -n flag

A router image always brings its own config. There is no user config, and so
the configs should always be overwritten!

Signed-off-by: Tim Niemeyer <tim.niemeyer@mastersword.de>

build_patches/sysupgrade_no_config_save.patch

@@ -0,0 +1,20 @@
+Index: package/base-files/files/sbin/sysupgrade
+===================================================================
+--- package/base-files/files/sbin/sysupgrade	(Revision 33502)
++++ package/base-files/files/sbin/sysupgrade	(Arbeitskopie)
+@@ -72,6 +72,15 @@
+ 	exit 1
+ }
+ 
++if [ "$SAVE_CONFIG" = "1" -a "$FORCE" = "0" ]; then
++    echo "Please let the image serve you with a new config."
++    echo "Use -n flag to get the new config!"
++    echo ""
++    echo "If you don't want to flash a Freifunk-Image you can force"
++    echo "a sysupgrade without -n using the -F parameter."
++    exit 1
++fi
++
+ [ -n "$ARGV" -a -n "$CONF_BACKUP" ] && {
+ 	cat <<-EOF
+ 		-b/--create-backup does not perform a firmware upgrade. Do not

buildscript

@@ -16,6 +16,9 @@ prepare() {
 	
 	$target/scripts/feeds install -a
 
+    # This changes the default behavior of sysupgrade to not save the config 
+    cat build_patches/sysupgrade_no_config_save.patch | patch -p0 -d $target
+
 	board_prepare
 }