lwip: bump to version v2.1.2

From lwIP CHANGELOG

(STABLE-2.1.2):

  ++ Bugfixes:

  2018-11-21: Jens Nielsen
  * netbiosns.c: fix expecting too large packet (bug #55069)

  2018-11-19: Dirk Ziegelmeier
  * smtp.c: fix compiling with strict C compatibility because of strnlen (bug #55034)

  2018-11-12: Simon Goldschmidt
  * tcp.c: fix overflow check in tcp_recved triggering invalid assertion (bug #55015)

  2018-11-12: Simon Goldschmidt
  * tcp.c: fix a bug in sending RST segments (sent from port 0)

(STABLE-2.1.1):

  ++ Bugfixes:

  2018-11-01: Joan Lledó
  * sockets.c: fix bad assertion in lwip_poll_dec_sockets_used() (bug #54933)

  2018-11-01: Dirk Ziegelmeier
  * ip4.c: don't send 127.* to default netif (bug #54670)

  2018-10-23: David Girault
  * altcp_tls_mbedtls.c: fix use-after free (bug #54774)

  2018-10-23: Ognjen Bjelica, Dirk Ziegelmeier
  * snmp_scalar.c: Avoid NULL pointer dereference (bug #54886)

  2018-10-23: Simon Goldschmidt
  * Fix missing standard includes in multiple files

  2018-10-17: Ivan Warren
  * def.h: fix casting htonX and ntohX to u16_t (bug #54850)

  2018-10-12: Simon Goldschmidt
  * Revert "tcp_abandon: no need to buffer pcb->local_port" (fix that source port was 0 for RST
    called when aborting a connection)

  2018-10-11: Jonas Rabenstein
  * tcp.c: tcp_recved: check for overflow and warn about too big values (patch #9699)

  2018-10-06: Joan Lledó
  * sockets.c: alloc_socket(): Check for LWIP_SOCKET_POLL when setting select-
    related variables (patch #9696)

  2018-10-04: Spencer
  * tcp.c: Update prev pointer when skipping entries in tcp_slowtmr (patch #9694)

  2018-09-27: Martine Lenders
  * lowpan6.c: Fix IEEE 802.15.4 address setting (bug #54749)

pkg/lwip/Makefile

@@ -1,7 +1,7 @@
 PKG_NAME=lwip
 PKG_URL=git://git.savannah.nongnu.org/lwip.git
-# lwIP v2.1.0
-PKG_VERSION=e6a8415df332ee34d7af02255b2aa1e8ee74348f
+# lwIP v2.1.2
+PKG_VERSION=159e31b689577dbf69cf0683bbaffbd71fa5ee10
 PKG_LICENSE=BSD-3-Clause
 
 LWIP_MODULES         = lwip_api lwip_core lwip_ipv4 lwip_ipv6 \