Implement grub_file tool and use it to implement generating of config

in separate root.

util/grub-mkconfig_lib.in

@@ -27,6 +27,9 @@ fi
 if test "x$grub_probe" = x; then
   grub_probe="${sbindir}/@grub_probe@"
 fi
+if test "x$grub_file" = x; then
+  grub_file="${bindir}/@grub_file@"
+fi
 if test "x$grub_mkrelpath" = x; then
   grub_mkrelpath="${bindir}/@grub_mkrelpath@"
 fi