pcrlock: don't fail if firmware measurements aren't available

With swtpm in place we now commonly have systems where TPM is available
during runtime, but not in the firmware. Handle that nicely.

src/pcrlock/pcrlock.c

@@ -899,6 +899,10 @@ static int event_log_load_firmware(EventLog *el) {
         path = tpm2_firmware_log_path();
 
         r = read_full_file(path, (char**) &buf, &bufsize);
+        if (r == -ENOENT) {
+                log_notice("No '%s' file, assuming TPM without firmware support.", path);
+                return 0;
+        }
         if (r < 0)
                 return log_error_errno(r, "Failed to open TPM2 event log '%s': %m", path);