[time] Use currticks() to provide the null system time

[time] Use currticks() to provide the null system time

For platforms with no real-time clock (such as RISC-V SBI) we use the
null time source, which currently just returns a constant zero.

Switch to using currticks() to provide a clock that does not represent
the real current time, but does at least advance at approximately the
correct rate.  In conjunction with the "ntp" command, this allows
these platforms to use time-dependent features such as X.509
certificate verification for HTTPS connections.

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