kbuild: Produce diff between base DT and U-Boot augmented DT if DEVICE_TREE_DEBUG=1

kbuild: Produce diff between base DT and U-Boot augmented DT if DEVICE_TREE_DEBUG=1

In case DEVICE_TREE_DEBUG is set, produce a diff between the base DT and
DT with U-Boot extras, to show how much does the U-Boot DT differ from
the base DT. This is particularly useful together with OF_UPSTREAM, to
minimize the diff between upstream DTs and U-Boot DTs.

This requires DTC 1.7.2 which does not resolve phandles when used in
the 'dtc -I dts -O dts ...' mode. With older DTC, the diff is full of
churn due to the resolved phandles.

Example usage:
$ make r8a779g3_sparrowhawk_defconfig && make DEVICE_TREE_DEBUG=1
$ cat ./dts/upstream/src/arm64/renesas/r8a779g3-sparrow-hawk.dtb.diff

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>