[Buildroot] Include kernel's dtsi from customised dts in buiuldroot-external

[Bruno Kremel <bruno.kremel@gmail.com>]

Hello All,

I have set up buildroot external for Toradex Colibri iMX6 board. I would
like to customise kernel default dts, so I have made a copy of this dts
to my external board/toradex/colibiri-imx6 and then in the defconfig I
set up the path to custom dts:
BR2_LINUX_KERNEL_USE_CUSTOM_DTS=y
BR2_LINUX_KERNEL_CUSTOM_DTS_PATH="$(BR2_EXTERNAL)/board/toradex/colibri-imx6/imx6dl-colibri-mcp-25.dts"

This successfully makes buildroot use my modified dts file. But this
file inludes some kernel dtsi files.
Namely:
#include <dt-bindings/input/input.h>
#include <dt-bindings/interrupt-controller/irq.h>
#include "imx6dl.dtsi"
#include "imx6qdl-colibri.dtsi"

These are however not found by dts compiler during build and I end up
with dtc failing on undefined symbols in devicetree.

How can I specify dts include path such that it will use kernel dtsi
files or what is the correct way to include these from external?

Thanks
Bruno