arm64/efi: Disable secure boot if shim is in insecure mode

arm64/efi: Disable secure boot if shim is in insecure mode

Port to arm64 a patch originally written by Josh Boyer for the x86 EFI
stub.

A user can manually tell the shim boot loader to disable validation of
images it loads.  When a user does this, it creates a UEFI variable called
MokSBState that does not have the runtime attribute set.  Given that the
user explicitly disabled validation, we can honor that and not enable
secure boot mode if that variable is set.

Signed-off-by: Linn Crosetto <linn@hpe.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Gbp-Pq: Topic features/all/securelevel
Gbp-Pq: Name arm64-efi-disable-secure-boot-if-shim-is-in-insecure.patch