Add lwIP and its CMSIS port as components

LwIP CMake build files require the presence of Doxygen to generate
documentation. If Doxygen isn't available on the build machine, CMake
doesn't finish the configuration.
To solve that issue, the SDK creates an empty lwipdocs target if Doxygen
isn't found.

Signed-off-by: Paul Szczepanek <paul.szczepanek@arm.com>