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=.

src/cryptsetup/cryptsetup.c

@@ -823,7 +823,7 @@ static int measure_volume_key(
                 return 0;
         }
 
-        r = efi_stub_measured(LOG_WARNING);
+        r = efi_measured_uki(LOG_WARNING);
         if (r < 0)
                 return r;
         if (r == 0) {