Add a test for a multiconfig build which mixes tiny and musl builds
along with using the mcextend class to combine and package multiple images
into another image. This gives the multiconfig a decent test in a scenario
users may use.
(From OE-Core rev: 0c7fa15a7350808242754944243f01155bc6784c)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>