To upgrade lwIP within the apache-mynewt-core repo:

1. Delete the old version of lwIP.  Remove everything in the `net/ip/lwip_base`
directory, *except* `MYNEWT` (this file) and `pkg.yml`:

    rm -r * && git checkout MYNEWT pkg.yml

2. Copy the lwIP repo contents into this directory (adjust source directory as
needed).

    cp -r ~/repos/lwip/* .

3. Move the `src/include` directory to the top-level:

    mv src/include .

4. Delete the Windows-only makefsdata app:

    rm -r src/apps/httpd/makefsdata

5. Delete the pppoe files (not Apache-license-compatible):

    rm include/netif/ppp/pppoe.h src/netif/ppp/pppoe.c