dts: Add ability to build fallback DTBOs from arch/$(ARCH)/dts

dts: Add ability to build fallback DTBOs from arch/$(ARCH)/dts

Currently the enablement of OF_UPSTREAM results on the build system
searching for DTs only in dts/upstream/ . There are platforms which
use U-Boot specific DTBOs applied on top of U-Boot control DT during
SPL stage, and source DTs for these are located in arch/$(ARCH)/dtb.

Add dedicated 'dtbos' target which builds only .dtbos and not .dtbs and
in case CONFIG_OF_UPSTREAM_INCLUDE_LOCAL_FALLBACK_DTBOS is enabled, build
this target for arch/$(ARCH)/dtb to generate local U-Boot specific DTBOs.

Adjust top level Makefile so binman would search for .dtb and .dtbo in
both OF_UPSTREAM specific paths and arch/$(ARCH)/dtb for the .dtbo case
in case CONFIG_OF_UPSTREAM_INCLUDE_LOCAL_FALLBACK_DTBOS is enabled.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>