Fix hex representation of binary variable contents

The getenv code was mishandling the conversion of binary to hex. Grub's
sprintf() doesn't seem to support the full set of format conversions, so
fix this in the nasty way.

grub-core/commands/efi/getenv.c

@@ -119,7 +119,7 @@ grub_cmd_getenv (grub_extcmd_context_t ctxt, int argc, char **args)
     {
       bindata = grub_zalloc(datasize * 2 + 1);
       for (i=0; i<datasize; i++)
-	  grub_snprintf(bindata + i*2, 3, "%02x", data[i]);
+	  grub_snprintf(bindata + i*2, 3, "%02x", data[i] & 0xff);
 
       if (grub_env_set (args[0], bindata))
 	goto done;