Matthew stopped maintaining the securelevel patch set, and David
Howells has taken it up under the new name 'lockdown'. This is
taken from:
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git#efi-lock-down
commits ddb99e118e37f324a4be65a411bb60ae62795cf9..0240fa7c7c948b19d57c0163d57e55296277ff3c
Rebase the three patches not included there (cold boot mitigation,
arm64 SB integration, MTD RAM restrictions).
Update our kconfig for the renaming.