cmake: ld: adding ld linker script generator

Adding intial version of ld_script.cmake.

This script can generate ld linker script from the Zephyr CMake linker
functions.

This script is called by the build system and all linker settings, such
as memory regions, groups, output sections, linker symbols defined with
Zephyr CMake linker functions are passed to this script in order to
generate an ld linker script.

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