kbuild: Don't assume dts files live in arch/*/boot/dts

In commit b40b25ff (kbuild: always run gcc -E on *.dts, remove cmd_dtc_cpp),
dts building was changed to always use the C preprocessor. This meant
that the .dts file passed to dtc is not the original, but the
preprocessed one.

When compiling with a separate build directory (i.e., with O=), this
preprocessed file will not live in the same directory as the original.
When the .dts file includes .dtsi files, dtc will look for them in the
build directory, not in the source directory and compilation will fail.

The commit referenced above tried to fix this by passing arch/*/boot/dts
as an include path to dtc. However, for mips, the .dts files are not in
this directory, so dts compilation on mips breaks for some targets.

Instead of hardcoding this particular include path, this commit just
uses the directory of the .dts file that is being compiled, which
effectively restores the previous behaviour wrt includes. For most .dts
files, this path is just the same as the previous hardcoded
arch/*/boot/dts path.

This was tested on a mips (rt3052) and an arm (bcm2835) target.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>