pkg: provide sock_udp support for lwip

Makefile.dep

pkg/lwip/Makefile.include

pkg/lwip/contrib/sock/udp/Makefile

pkg/lwip/contrib/sock/udp/lwip_sock_udp.c

pkg/lwip/include/sock_types.h

tests/lwip_sock_udp/Makefile

tests/lwip_sock_udp/README.md

tests/lwip_sock_udp/constants.h

tests/lwip_sock_udp/main.c

tests/lwip_sock_udp/stack.c

tests/lwip_sock_udp/stack.h

tests/lwip_sock_udp/tests/01-run.py