oe-selftest: add new ext dtb recipe

oe-selftest: add new ext dtb recipe

There is no test coverage for the devicetree.bbclass class. Add a
minimalist recipe that uses this class.
This recipe compiles a devicetree overlay that does not include any
sources from the kernel build folder to keep it simple and also usable
for testing a kernel from the sstate-cache.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>