[settings] Add "unixtime" builtin setting to expose the current time

[settings] Add "unixtime" builtin setting to expose the current time

Expose the current wall-clock time (in seconds since the Epoch), since
this is often useful in captured boot logs and can also be useful when
checking unexpected X.509 certificate validation failures.

Use a :uint32 setting to avoid Y2K38 rollover, thereby ensuring that
this will eventually be somebody else's problem.

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