cmake: kconfig: support for multiple SOC_ROOT

This commit introduces support for multiple SOC_ROOT.

This means that additional SOC_ROOTs specified using -DSOC_ROOT as
argument to CMake will be forming a list together with ${ZEPHYR_BASE}.

This allows for greater flexibility, as developers can now specify
multiple out-of-tree SoCs and not worry about the SoC used for the
board they compile for.

Also it avoid code, such as:
if(BOARD STREQUAL my_board_using_out_of_tree_soc)
  set(SOC_ROOT some/out/of/tree/soc/path)
endif()
in application CMakeLists.txt.

Finally, allowing multiple SOC_ROOTs prepares for specifying SOC_ROOTs
in Zephyr modules.

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