[crypto] Allow an error margin on X.509 certificate validity periods

[crypto] Allow an error margin on X.509 certificate validity periods

iPXE has no concept of the local time zone, mainly because there is no
viable way to obtain time zone information in the absence of local
state.  This causes potential problems with newly-issued certificates
and certificates that are about to expire.

Avoid such problems by allowing an error margin of around 12 hours on
certificate validity periods, similar to the error margin already
allowed for OCSP response timestamps.

Signed-off-by: Michael Brown <mcb30@ipxe.org>