cert-backup: Mode to extract a file

While the files are meant to be extracted during the factory reset, it
may be desired to extract a specific file at other occasion as well.
Provide the possibility.

cert-backup/README

@@ -11,3 +11,7 @@ For example, to add files /etc/a and /etc/b and remove /etc/c and /etc/d, you'd
 call it as:
 
   ./cert-backup /etc/a /etc/b -r /etc/c -r /etc/d
+
+In addition to -r, a file name may be prefixed with -x. In such case, the file
+is extracted from the backup, not stored. As with -r, -x modifies behaviour for
+only the single following file.

cert-backup/cert-backup

 #!/bin/sh
 
-# Copyright (c) 2013, CZ.NIC, z.s.p.o. (http://www.nic.cz/)
+# Copyright (c) 2013-2015, CZ.NIC, z.s.p.o. (http://www.nic.cz/)
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -38,6 +38,10 @@ while [ "$1" ] ; do
 		# It should be removed, if it exists
 		shift
 		rm -f "$WORKDIR"/"$1"
+	elif [ "$1" = '-x' ] ; then
+		# Reverse, extractor mode
+		shift
+		cp -a "$WORKDIR"/"$1" "$1"
 	elif cmp -s "$1" "$WORKDIR"/"$1" ; then
 		echo "$1" already stored, skipping
 	else