build.sh: split build steps for platform lwip

Apparently, the vpath magic in the lwip examples' Makefile requires
the lwip files to exist before the server is built. Thus, the build
script now calls make twice: First, to clone the lwip repositories,
and second, to create the actual server application.

scripts/build.sh

@@ -14,6 +14,7 @@ case "${PLATFORM}" in
                make -C examples/contiki
              ;;
     lwip)    config "--disable-tests --disable-documentation --disable-examples" && \
+               make -C examples/lwip lwip lwip-contrib
                make -C examples/lwip
              ;;
     posix|*) config "$WITH_TESTS --enable-documentation --enable-examples" && \