netInterface is set down when in DHCP/BOOTP mode and link up event occurs

The current implementation is inconsistent in the way it uses the interface status (IF-UP / IF-DOWN) when executing the DHCP/BOOT client functionality.

When starting DHCP (discover) the IP address is set to zero internally and the interface is set to IF-UP.
In case there is a link up event, the DHCP process is restarted with IP address being set to 0, but setting the Interface state to IF_DOWN.

Therefore, the stack behaves differently depending on whether or not a link up event occurred.