cmake: modules: add pre_dt

Create a separate module that sets up all our devicetree handling, by
setting up common variables that would apply to any and all DT
processing. This is then included in the regular dts module that we
include in zephyr_default.cmake.

The separation of this code from dts.cmake is groundwork for enabling
system devicetree in Zephyr, which will need the same definitions
included into its scope.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>