[LIBS] netstack: Enable core locking in lwIP for improved performance

By default, lwIP uses "tcpip_thread" thread for serializing Rx, Tx and
various other requests. This makes lwIP multi-threading safe but comes
with a price of reduced performance.

We also have experimental "core locking" options in lwIP which if enabled
causes lwIP to use a global mutex lock for serializing Rx, Tx and other
requests. It is also known fact that "core locking" does improve lwIP
performance but it is still experimental.
(Refer, https://lists.gnu.org/archive/html/lwip-users/2011-07/msg00068.html)

This patch enables "core locking" for lwIP.

Signed-off-by: Anup Patel <anup@brainfault.org>