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)

src/bootctl/bootctl-status.c

@@ -391,6 +391,7 @@ int verb_status(int argc, char *argv[], void *userdata) {
                         { EFI_LOADER_FEATURE_MENU_DISABLE,            "Menu can be disabled"                  },
                         { EFI_LOADER_FEATURE_MULTI_PROFILE_UKI,       "Multi-Profile UKIs are supported"      },
                         { EFI_LOADER_FEATURE_REPORT_URL,              "Loader reports network boot URL"       },
+                        { EFI_LOADER_FEATURE_TYPE1_UKI,               "Support Type #1 uki field"             },
                 };
                 static const struct {
                         uint64_t flag;