boot: add new bls type #1 stanza "uki"

boot: add new bls type #1 stanza "uki"

This one is between "efi" and "linux": we'll recognize such entries as
linux, but we'll just invoke them as EFI binaries.

This creates a high-level concept for invoking UKIs via indirection of a
bls type #1 entry, for example to permit invocation from a non-standard
path or for giving entries a different name.

Companion BLS spec PR:

https://github.com/uapi-group/specifications/pull/135

(Let's rename LOADER_UNIFIED_LINUX to LOADER_TYPE2_UKI at the same time
to reduce confusion what is what)