[parseopt] Add parse_timeout()

Parsing a timeout value (specified in milliseconds) into an internal
timeout value measured in timer ticks is a common operation.  Provide
a parse_timeout() value to carry out this conversion automatically.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[netdevice] Make all net_driver methods optional

Most network upper-layer drivers do not implement all three methods
(probe, notify, and remove).  Save code by making all methods
optional.

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