ipq806x: Point to externally compiled dtbs in recipes

ipq806x: Point to externally compiled dtbs in recipes

Similar to commit 4d8b42d8a777 ("ipq40xx: point to externally compiled
dtbs in recipes").

Currently, we patch our DTS files into the kernel source tree, so the
kernel build process will produce DTBs for us. The kernel-to-DTS
dependency can cause buildroot to perform excessive rebuilds of the
kernel though, which slows down device development iteration.

Buildroot also compiles DTBs on its own, to
$(KDIR)/image-$(DEVICE_DTS).dtb. With small adjustments, we can leverage
this, and stop patching DTS files into the kernel Makefile at the same
time.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>