Update to lwIP 2.1.2

Update to lwIP 2.1.2

Update lwIP from 2.0.3 to the latest relase 2.1.2 from Dec 2018. We
transform the rather beta state of our glue library to a more stable
implementation based on the latest staging branch of Unikraft towards
release 0.3.

This commit provides the following features as selectable options from
lwIP:
- Two operation modes: "threaded" or as (part of a) "mainloop"
- IPv4 support
- Experimental IPv6 support (incomplete)
- Protocols: UDP, TCP, ICMP, IGMP, SNMP
- DNS resolver
- DHCPv4 client
- Socket API

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Signed-off-by: Florian Schmidt <florian.schmidt@neclab.eu>
Signed-off-by: Yuri Volchkov <yuri.volchkov@neclab.eu>
Reviewed-by: Sharan Santhanam <sharan.santhanam@neclab.eu>