buildroot

atest: new package

Add support for atest application, which is useful for testing
alsa kernel drivers and detecting if playback/capture work correct
without artifacts, such as channel swap.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
[Thomas: add missing Config.in comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>