setup-disk: add magic environemnt var ERASE_DISKS as confirmation

Add a magic environment variable ERASE_DISKS as confirmation for
erasing disks. This is useful for scripted installs.

fixes #2863

setup-disk.in

@@ -437,6 +437,10 @@ show_disk_info() {
 
 confirm_erase() {
 	local answer=
+	local erasedisks="$@"
+	if [ "$ERASE_DISKS" = "$erasedisks" ]; then
+		reeturn 0
+	fi
 	echo "WARNING: The following disk(s) will be erased:"
 	show_disk_info $@
 	echo -n "WARNING: Erase the above disk(s) and continue? [y/N]: "