net: lwip: add DHCP support and dhcp commmand

net: lwip: add DHCP support and dhcp commmand

Add what it takes to enable NETDEVICES with NET_LWIP and enable DHCP as
well as the dhcp command. CMD_TFTPBOOT is selected by BOOTMETH_EFI due
to this code having an implicit dependency on do_tftpb().

Note that PXE is likely non-fonctional with NET_LWIP (or at least not
100% functional) because DHCP option 209 is not supported by the lwIP
library. Therefore, BOOTP_PXE_DHCP_OPTION cannot be enabled.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>