net: lwip: build lwIP

net: lwip: build lwIP

Build the lwIP library when NET_LWIP is enabled. The following files
are adaptation layers written specially for U-Boot:

 lib/lwip/u-boot/arch/cc.h
 lib/lwip/u-boot/arch/sys_arch.h (empty)
 lib/lwip/u-boot/limits.h (empty)
 lib/lwip/u-boot/lwipopts.h

They were initially contributed by Maxim in a previous RFC patch series.

The lwIP stack needs to be added as a Git subtree with the following
command:

 $ git subtree add --squash --prefix lib/lwip/lwip \
   https://github.com/lwip-tcpip/lwip.git  STABLE-2_2_0_RELEASE

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Co-developed-by: Maxim Uvarov <muvarov@gmail.com>
Cc: Maxim Uvarov <muvarov@gmail.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>