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.

platform/lwip: Fix freertos NO_SYS=0 compile issue

If NO_SYS=0 it uses freertos stuff like xQueueCreate, so needs to
include the headers.

bnep_lwip: return bool in bnep_lwip_outgoing_packets_empty

bnep_lwip: use generic btstack_run_loop_execute_on_main_thread

license: update text

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