[settings] Allow system time to be modified via builtin/unixtime

Allow the system time offset to be modified by writing a new time
value to builtin/unixtime, e.g.:

  set builtin/unixtime 0x10d1a884

As with the NTP client, this does not attempt to write to the
underlying clock source (e.g. the RTC clock).  Only the internal
system time offset is updated.

Any system time offset may be reset by clearing the setting:

  clear builtin/unixtime

This will reset the system time offset to zero and so can be used to
undo the effect of a previous "set builtin/unixtime" or "ntp" command.

Requested-by: Christian I. Nilsson <ChristianN@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>