Add option to automatically set securelevel when in Secure Boot mode

Add option to automatically set securelevel when in Secure Boot mode

UEFI Secure Boot provides a mechanism for ensuring that the firmware will
only load signed bootloaders and kernels. Certain use cases may also
require that the kernel prevent userspace from inserting untrusted kernel
code at runtime. Add a configuration option that enforces this automatically
when enabled.

Signed-off-by: Matthew Garrett <mjg59@srcf.ucam.org>
Gbp-Pq: Topic features/all/securelevel
Gbp-Pq: Name add-option-to-automatically-set-securelevel-when-in-.patch