The newer btrfs-utils needs an empty file to build the filesystem in, so
create an empty file and use it for the mkfs to build the fs in.
[YOCTO #6804]
(From OE-Core rev: afc44fad44261677c799558ffd35f4908556bce0)
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>