cmake: Introduce Zephyr interface libraries

Introduce Zephyr interface libraries to fix #5351.

From the documentation:

A Zephyr interface library is a thin wrapper over a CMake INTERFACE
library. The most important responsibility of this abstraction is to
ensure that when a user KConfig-enables a library then the header
files of this library will be accessible to the 'app' library.

This is done because when a user uses Kconfig to enable a library he
expects to be able to include it's header files and call it's
functions out-of-the box.

A Zephyr interface library should be used when there exists some
build information (include directories, defines, compiler flags,
etc.) that should be applied to a set of Zephyr libraries and 'app'
might be one of these libraries.

Zephyr libraries must explicitly call
zephyr_library_link_libraries(<interface_library>) to use this build
information. 'app' is treated as a special case for usability
reasons; a Kconfig option (CONFIG_APP_LINK_WITH_<interface_library>)
should exist for each interface_library and will determine if 'app'
links with the interface_library.

This API has a constructor like the zephyr_library API has, but it

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>