dl: Add support for persistent modules

This type of modules cannot be unloaded. This is useful if a given
functionality, e.g. UEFI secure boot shim signature verification, should
not be disabled if it was enabled at some point in time. Somebody may
say that we can use standalone GRUB2 here. That is true. However, the
code is not so big nor complicated hence it make sense to support
modularized configs too.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>

grub-core/commands/minicmd.c

@@ -137,6 +137,9 @@ grub_mini_cmd_rmmod (struct grub_command *cmd __attribute__ ((unused)),
   if (! mod)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, "no such module");
 
+  if (grub_dl_is_persistent (mod))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "cannot unload persistent module");
+
   if (grub_dl_unref (mod) <= 0)
     grub_dl_unload (mod);