bbfdm: add udhcpc.user.d script to store dhcp lease start time

DCHP lease start time is needed to calculate remaining lease time which is needed for Device.DHCPv4.Client.{i}.LeaseTimeRemaining.

Earlier this was being provided via interface.status, but that was leading to unnecessary hotplug events for interface data update so it had to be removed from there.

Add the logic in udhcpc.user.d script separate from netifd as there does not seem to be a need in netifd for this information.