ipq40xx: point to externally compiled dtbs in recipes

ipq40xx: point to externally compiled dtbs in recipes

Adjusting dts will cause a rebuild of whole kernel as the buildroot
considers this a part of kernel source. It's a royal PITA when trying to
prepare support for new device, since this takes a lot of time on slower
systems. As it stands, buildroot itself, with own rule, also compiles
dtbs and the results are $(KDIR)/image-$(DEVICE_DTS).dtb. With setting
DEVICE_DTS_DIR to directory holding the device dts (similarly to some
other targets), buildroot doesn't consider changed dts as part of kernel
source and rebuilds only dtb. This really speeds up development. And
since the kernel built dts are no longer used, drop the paches adding
dtses to its build.

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Reviewed-by: Robert Marko <robimarko@gmail.com>