net: make dhcp_run() common for NET and NET_LWIP

net: make dhcp_run() common for NET and NET_LWIP

There are currently two implementations of dhcp_run(): one in cmd/net.c
for NET and one in net/lwip/dhcp.c for NET_LWIP. There is no
justification for that. Therefore, move the NET version into
net/net-common.c to be used by both stacks, and drop the NET_LWIP
version which by the way does not look totally correct.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Acked-by: Benjamin Hahn <B.Hahn@phytec.de>