liblwip: Exclude lwipopts.h

Including the library's version of lwipopts.h config prevents users from
using a custom version of the lwipopts.h config file for each
application and instead restricts the user to having to modify the
library's version of lwipopts.h which isn't a suitable workflow.

This commit excludes the library's version of lwipopts.h from the build.
Users are now required to use the CMake macro inside the
lwip_helpers.cmake file to include a custom version of lwipopts.h. Also
update style filter to point to the new default lwipopts.h location.

liblwip: Fix lwIP search path

The CMAKE_SOURCE_DIR is not always in the root of the entire project
i.e. where '.repo' is located, so we instead use a relative path from
the current CMakeLists.txt directory.

liblwip: Update to use latest lwIP sources

This commit updates the CMakeLists.txt scripts to use the latest lwIP
sources instead of the sources bundled with the "library". Also update
the lwipopts.h options header and add extra configuration values.

replace kernel_autoconf with sel4_autoconf

since the mainline kernel's kernel_autoconf doesn't include
seL4/gen_config.h but sel4_autoconf does

libethdrivers: include config of lwip and picotcp

since global autoconf is removed, we need to explicitly
include the config of lwip and picotcp which will further
indicate the existance of corresponding lib

util_libs: remove autoconf.h from util_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

CMake: Mark config options as advanced

This is to hide the options in the cmake-gui. Top level applications can
choose to reexpose these options by clearing the advanced setting.

trivial: style CMakeLists.txt

liblwip: CMake support for liblwip

Added CMake support for liblwip. Added a CMakeLists file for the
liblwip directory. liblwip is dependent on the interface library
'liblwip_config'. A project using liblwip is expected to provide
it owns lwipopts.h config header, provided through the
lwip_config library.