shim_lock: Only skip loading shim_lock verifier with explicit consent

shim_lock: Only skip loading shim_lock verifier with explicit consent

Commit 32ddc42c (efi: Only register shim_lock verifier if shim_lock
protocol is found and SB enabled) reintroduced CVE-2020-15705 which
previously only existed in the out-of-tree linuxefi patches and was
fixed as part of the BootHole patch series.

Under Secure Boot enforce loading shim_lock verifier. Allow skipping
shim_lock verifier if SecureBoot/MokSBState EFI variables indicate
skipping validations, or if GRUB image is built with --disable-shim-lock.

Fixes: 132ddc42c (efi: Only register shim_lock verifier if shim_lock
       protocol is found and SB enabled)
Fixes: CVE-2020-15705
Fixes: CVE-2021-3418
Reported-by: Dimitri John Ledkov <xnox@ubuntu.com>
Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>