Add a FIT_DESC variable to make it possible to change how the
description is set in the FIT image.
(From OE-Core rev: 47c5ea69e1a6c4fd3aa766d5223aff1201a4a1d8)
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>