Allow TCP stack to be initialized with static configuration and DHCP

[karlklose]

Currently we support a mixed mode, where we always use DHCP and a routable static IP address as the fallback. This makes it rather complicated to reason about the state of the network interface and stack (see discussion in https://codereview.chromium.org/2024143003/).

Instead, the network library should support two ways to initialize the stack:

1. provide a static IP. This IP is always assigned to the network interface, even if there is no physical connection.
2. DHCP. No IP address is assigned to the network interface until a valid DHCP configuration has been received and after the lease has expired.