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>

docs/grub.texi

@@ -3841,6 +3841,11 @@ If this variable is set and true (i.e., not set to ``0'', ``false'',
 fatal. Otherwise, they will merely be debug-logged and boot will
 continue.
 
+Call to EFI firmware, like hash_log_extend_event(), can return an unknown
+error, i.e. due to bug present in firmware. When this variable is set and
+true (same values as with TPM measurements) this situation will be considered
+to be fatal and error-logged as ``unknown TPM error''. If not set, booting
+the OS will be enabled.
 
 @node Environment block
 @section The GRUB environment block