uki.bbclass: add class for building Unified Kernel Images (UKI)

This class calls systemd ukify tool, which will combine
kernel/initrd/stub components to build the UKI. To sign the UKI
(i.e. SecureBoot), the keys/cert files can be specified
in a configuration file or UEFI binary signing can be done
via separate steps, see qemuarm64-secureboot in meta-arm.
UKIs are loaded by UEFI firmware on target which can improve
security by loading only correctly signed kernel, initrd and kernel
command line.

Using systemd-measure to pre-calculate TPM PCR values and sign them is
not supported since that requires a TPM device on the build host. Thus
"ConditionSecurity=measured-uki" default from systemd 256 does not work
but "ConditionSecurity=tpm2" in combination with secure boot will.
These can be used to boot securely into systemd-boot, kernel, kernel
command line and initrd which then securely mounts a read-only dm-verity
/usr partition and creates a TPM encrypted read-write / rootfs.

Tested via qemuarm64-secureboot in meta-arm with
https://lists.yoctoproject.org/g/meta-arm/topic/patch_v3_02_13/108031399
and a few more changes needed, will be posted separately.

(From OE-Core rev: da69a73491a72b46e299f874ce90d81135cd9656)

Signed-off-by: Michelle Lin <michelle.linto91@gmail.com>
Acked-by: Erik Schilling <erik.schilling@linaro.org>
Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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