cmake: allowing list of ARCH_ROOTs

This commit aligns ARCH_ROOT to conform to BOARD_ROOT, DTS_ROOT,
SOC_ROOT that all allows list of directories.

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

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