platform/lwip: add getter for netif provided by bnep

BNEP adds an interface to lwip. Add an accessor to allow us to get
access to it.

license: update text

bnep_lwip: add bnep_lwip_connect to establish BNEP connection and manage lwIP network interface

new pan_lwip_http_server example

- add lwIP sources commit b3a939417 (after STABLE-2.1.1) as 3rd-party/lwip/core
- add small DHCP server as 3rd-party/lwip/dhcp-server
- add BNEP lwIP adapter that ha

new pan_lwip_http_server example

- add lwIP sources commit b3a939417 (after STABLE-2.1.1) as 3rd-party/lwip/core
- add small DHCP server as 3rd-party/lwip/dhcp-server
- add BNEP lwIP adapter that handles BNEP events and forwards packets between BNEP and lwIP (only tested for NO_SYS == 1)
- add pan_lwip_http_server example that provides lwIP http server on http://192.168.7.1
- example is added to port/libusb

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