lib/packet: DHCP packet parser/serializer

Add DHCP encoder/decoder class.

Signed-off-by: TAKESHITA Noboru <takeshita.noboru@yes.nttcom.ne.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Add Packet library API Reference

Add DHCP encoder/decoder class.

Signed-off-by: TAKESHITA Noboru <takeshita.noboru@yes.nttcom.ne.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>