cmake: update board and soc linker script handling

This commit updates the handling of board and SoC linker scripts.

Several SoCs creates a linker.ld file which sole purpose is to include
another arch common linker script, often with content like this:

    #include <arch>/linker.ld

instead of 100+ SoC specific linker.ld files containing just a single
include line of above structure, then this commit introduces two now
CMake variables, BOARD_LINKER_SCRIPT and SOC_LINKER_SCRIPT.

This allows the board and SoC CMake code to point directly to a common
linker script instead of creating a dummy linker.ld file doing this.

This removes the need for several dummy linker.ld file.

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