libplatsupport: Add tools to gen device headers

The new functions allow the user to input a YAML file that contains a
list of sub-devices of a particular device (GPIO pins, clocks on a
board etc), and output a C header '#define' enumeration.

CMake: Add LWIP_HELPERS to util_libs package

This saves the path of the lwip_helpers.cmake file.

CMake: Add Findutil_libs.cmake module

This adds support for importing this project via
`find_package(util_libs)` if the project exists in the
CMAKE_MODULE_PATH.