MODSIGN: check the attributes of db and mok

MODSIGN: check the attributes of db and mok

That's better for checking the attributes of db and mok variables
before loading certificates to kernel keyring.

For db and dbx, both of them are authenticated variables. Which
means that they can only be modified by manufacturer's key. So
the kernel should checks EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS
attribute before we trust it.

For mok-rt and mokx-rt, both of them are created by shim boot loader
to forward the mok/mokx content to runtime. They must be runtime-volatile
variables. So kernel should checks that the attributes map did not set
EFI_VARIABLE_NON_VOLATILE bit before we trust it.

Cc: David Howells <dhowells@redhat.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
[Rebased by Luca Boccassi]
[bwh: Forward-ported to 5.0: adjust filename, context]

Gbp-Pq: Topic features/all/db-mok-keyring
Gbp-Pq: Name 0004-MODSIGN-check-the-attributes-of-db-and-mok.patch