dl: Provide a fake grub_dl_set_persistent() and grub_dl_is_persistent() for the emu...

dl: Provide a fake grub_dl_set_persistent() and grub_dl_is_persistent() for the emu target

Trying to start grub-emu with a module that calls grub_dl_set_persistent()
and grub_dl_is_persistent() will crash because grub-emu fakes modules and
passes NULL to the module init function.

Provide an empty function for the emu case.

Fixes: ee7808e2197c (dl: Add support for persistent modules)
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>