Import new lwIP version into liblwip

In order to match NetBSD-style imports of external code, the library
has been restructured. The full lwIP source tree is imported, except
for a few .git* files

Import new lwIP version into liblwip

In order to match NetBSD-style imports of external code, the library
has been restructured. The full lwIP source tree is imported, except
for a few .git* files in its root directory, into dist/. The MINIX 3
Makefiles and other custom files are located in lib/. Finally, since
we need to apply a number of small patches to lwIP, these patches are
stored in patches/, in addition to being applied to the lwIP tree.

The currently imported version of lwIP is taken from its master
branch sometime after the 2.0.1 release, specifically git-7ffe5bf.

Change-Id: Ie03c4fa36fa928870263c191205d6d93f652a3cc

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