[time] Allow system clock to be adjusted at runtime

[time] Allow system clock to be adjusted at runtime

Provide a mechanism to allow an arbitrary adjustment to be applied to
all subsequent calls to time().

Note that the underlying clock source (e.g. the RTC clock) will not be
changed; only the time as reported within iPXE will be affected.

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