cmake: re-work devicetree preprocessing steps

This causes any devicetree error messages to refer to the source files
which contain the errors instead of intermediate <BOARD>.dts.pre.tmp
files in the build directory.

Do this using a new python-devicetree feature which can correctly
handle cpp's generated line marker directives.

To use this feature, rework the way that the C preprocessor is run on
devicetree inputs so that line marker directives are preserved by
removing the -P option.

This is a backwards incompatible change to the way the generated files
in the build directory used to work, as not all tools can consume line
markers. In particular, dtc can't handle these lines. We therefore
pass dtc zephyr.dts instead (the final parsed devicetree output from
python-devicetree).

Since <BOARD>.dts.pre.tmp is a publicly documented file, this is
changing existing behavior, so use a new file name for the
intermediate files to make it more obvious that something changed. In
particular, use zephyr.dts.pre instead of <BOARD>.dts.pre.tmp.
(The $BOARD.dts.pre.tmp name is a little cumbersome anyway.)

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