This patch is taken from OpenWrt, see:

https://github.com/openwrt/openwrt/blob/master/package/boot/grub2/patches/100-grub_setup_root.patch.

It allows to use grub-bios-setup on a raw disk-images, without root permissions.

--- a/util/grub-setup.c
+++ b/util/grub-setup.c
@@ -87,6 +87,8 @@ static struct argp_option options[] = {
    N_("install even if problems are detected"), 0},
   {"skip-fs-probe",'s',0,      0,
    N_("do not probe for filesystems in DEVICE"), 0},
+  {"root-device", 'r', N_("DEVICE"), 0,
+   N_("use DEVICE as the root device"), 0},
   {"verbose",     'v', 0,      0, N_("print verbose messages."), 0},
   {"allow-floppy", 'a', 0,      0,
    /* TRANSLATORS: The potential breakage isn't limited to floppies but it's
@@ -130,6 +132,7 @@ struct arguments
   char *core_file;
   char *dir;
   char *dev_map;
+  char *root_dev;
   int  force;
   int  fs_probe;
   int allow_floppy;
@@ -178,6 +181,13 @@ argp_parser (int key, char *arg, struct
         arguments->dev_map = xstrdup (arg);
         break;
 
+      case 'r':
+        if (arguments->root_dev)
+          free (arguments->root_dev);
+
+        arguments->root_dev = xstrdup (arg);
+        break;
+
       case 'f':
         arguments->force = 1;
         break;
@@ -313,7 +323,7 @@ mai</td></tr></table>
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