grub2: CVE-2022-28735 shim_lock verifier allows non-kernel files to be loaded

Upstream-Status: Backport from https://git.savannah.gnu.org/cgit/grub.git/commit/?id=6fe755c5c07bb386fda58306bfd19e4a1c974c53



Signed-off-by: Hitendra Prajapati <hprajapati@mvista.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>