selftest/multiconfig: Test that multiconfigs in separate layers works

We should test that mutliconfigs from a layer work, not just build/conf.
This adds such a test.

[YOCTO #13566]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>