OLPC trac 4707 - allow secure booting even if the real time clock is
set incorrectly. When that is the case, all leases are deemed to have
expired, but things like developer keys still work.
Also added support for developer key expiration. If the expiration date
on either a lease or a developer signature is the value 00000000T000000Z,
the signature doesn't expire and is always deemed timely. Otherwise it
is compared against the current system time.