rc: employ the same write probe trick for early installer menu

src/etc/rc.bootup

@@ -29,6 +29,24 @@
 	POSSIBILITY OF SUCH DAMAGE.
 */
 
+function is_install_media()
+{
+	/*
+	 * Despite unionfs underneath, / is still not writeable,
+	 * making the following the perfect test for install media.
+	 */
+
+	$file = '/.probe.for.install.media';
+	$fd = @fopen($file, 'w');
+	if ($fd) {
+		fclose($fd);
+		unlink($file);
+		return false;
+	}
+
+	return true;
+}
+
 function rescue_detect_keypress()
 {
 	$timeout = 9;
@@ -141,8 +159,8 @@ set_device_perms();
 unmute_kernel_msgs();
 echo "done.\n";
 
-// Display rescue configuration option
-if ($g['platform'] == 'cdrom') {
+/* Display live system's early boot options */
+if (is_install_media()) {
 	rescue_detect_keypress();
 }