cmake: support sub-component loading in zephyr_default module

This commit allows to load only subcomponents of the default Zephyr
CMake module, zephyr_default.cmake.

This allows other tools to execute Zephyr CMake build system up to a
specific module and the stop further processing.

This commit is an enabler for future support in twister to process only
devicetree or kconfig, to allow test / sample filtering before
generating a complete build system.

Sub-components can be loaded as:
> find_package(Zephyr COMPONENTS zephyr_default:<sub-component>)

for example:
> find_package(Zephyr COMPONENTS zephyr_default:dts)

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