Make the check for tap devices available as a function which can be used
by other tests as well.
(From OE-Core rev: ad8f3a8d959a245301118cf7b850f1a0ab567f01)
Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>