tpm: Enable boot despite unknown firmware failure

Currently booting the system is prevented when call to EFI firmware
hash_log_extend_event() returns unknown error. Solve this by following
convention used in commit a4356538d (commands/tpm: Don't propagate
measurement failures to the verifiers layer).

Let the system to be bootable by default when unknown TPM error is
encountered. Check environment variable tpm_fail_fatal to fallback to
previous behaviour.

Signed-off-by: Michał Grzelak <mchl.grzlk@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/efi/tpm.c

@@ -146,7 +146,7 @@ grub_efi_log_event_status (grub_efi_status_t status)
     case GRUB_EFI_NOT_FOUND:
       return grub_error (GRUB_ERR_UNKNOWN_DEVICE, N_("TPM unavailable"));
     default:
-      return grub_error (GRUB_ERR_UNKNOWN_DEVICE, N_("unknown TPM error"));
+      return grub_error (grub_is_tpm_fail_fatal () ? GRUB_ERR_UNKNOWN_DEVICE : GRUB_ERR_NONE, N_("unknown TPM error"));
     }
 }