cmake: ld: adding common linker script generator

Adding intial version of linker_script_common.cmake.

This script parses memory regions, groups, sections, settings, and
symbols defined in the Zephyr CMake build system.

The linker script creates objects for each type above and groups
sections, symbols, and groups together based on their configuration.

This creates a hierarchy which the `<linker>_script.cmake`
implementations can then use during linker script generation.

linker_script_common.cmake also provides stubs for <type>_to_string()
functions which are used when generating the output script.

Each specific `<linker>_script.cmake` must implement those to_string()
functions to create the final linker script.

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