cmake: zephyr_library_amend feature

This commit introduces the cmake extension zephyr_library_amend.

This function allows for adding files in an out-of-tree Zephyr module
to a zephyr library created in zephyr repo CMake files.

As example:
drivers/entropy/CMakeLists.txt creates an zephyr library as:
zephyr_library()
only available to zephyr itself.

The amend function allows to amend to such a lib, by creating a
CMakeLists.txt file following identical folder structure in a Zephyr
Module:
<zephyr_module_oot>/drivers/entropy/CMakeLists.txt
zephyr_library_amend()
zephyr_library_sources() # Sources are amended to the original library

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>