This adds support for UEFI Secure Boot. It adds the missing pieces to the Linux
Kernel and enforces module signing. This results in an additional security
layer where untrusted (unsigned) Kernel modules can no longer be loaded into
the live system.
NOTE: This commit will not work unless signing keys are present. Arbitrary
keys can be generated using instructions found in:
data/live-build-config/includes.chroot/var/lib/shim-signed/mok/README.md
... Kernel is appended a + to the version string when there are locally
modified files - which we have. This is prevented by the existence of the
.scmversion file.
