BCM270X_DT: Add a .dtbo target, use for overlays

Change the filenames and extensions to keep the pre-DDT style of
overlay (<name>-overlay.dtb) distinct from new ones that use a
different style of local fixups (<name>.dtbo), and to match other
platforms.

The RPi firmware uses the DDTK trailer atom to choose which type of
overlay to use for each kernel.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>