Merge branch 'features/lwip-SO_REUSEADDR' into 'master'

Enable SO_REUSEADDR in LWIP

Daniel initially asked me why this wasn't enabled, and I don't think I got any reasons against enabling this. If any, it makes porting existing software easier. Tuan needs it for UDP multicast as well.

Code changes are by Tuan: basically the enable for SO_REUSEADDR in LWIP as well as a bugfix in a bit of mbedTLS that gets enabled.

See merge request !90