kbuild: Allow .dtbo overlays to be built piecemeal

Before 4.20, it was possible to build an arbitrary overlay by copying
it to arm/boot/dts/overlays/mytest-overlay.dts and running:

    make ARCH=arm overlays/mytest.dtbo

In 4.20 the .dtb build rules were centralised, requiring the dowstream
.dtbo build rules to be changed. They were, enough to support "make ...
dtbs", but not sufficiently to allow this ad-hoc, one-off building of
individual files.

Add the missing makefile rule to support this way of building.

See: https://github.com/raspberrypi/linux/issues/3250



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