tpm2-util: load external key into NULL hierarchy if private key is provided

If we load an external key into the TPM we must do so in the NULL
hierarchy. An external key after all is one that is not wrapped by any
hierarchy's seed.

See TPM2 spec, Part 3, Section 12.3.1

src/shared/tpm2-util.c

@@ -2242,9 +2242,9 @@ static int tpm2_load_external(
 #if HAVE_TSS2_ESYS3
                         /* tpm2-tss >= 3.0.0 requires a ESYS_TR_RH_* constant specifying the requested
                          * hierarchy, older versions need TPM2_RH_* instead. */
-                        ESYS_TR_RH_OWNER,
+                        private ? ESYS_TR_RH_NULL : ESYS_TR_RH_OWNER,
 #else
-                        TPM2_RH_OWNER,
+                        private ? TPM2_RH_NULL : TPM2_RH_OWNER,
 #endif
                         &handle->esys_handle);
         if (rc != TSS2_RC_SUCCESS)