efi-loader: rename efi_stub_measured() → efi_measured_uki()

efi-loader: rename efi_stub_measured() → efi_measured_uki()

Let's say "uki" rather than "stub", since that is just too generic, and
we shouldn't limit us to our own stub anyway, but generally define a
concept of a "measured UKI", which is a UKI that measures its part to
PCR 11.

This is mostly preparation for exposing this check to the user via
ConditionSecurity=.