direct.py: fix getting image name
part.rootfs_dir was used as an image name in the code.
However, when multi-rootfs feature is used this attribute
points to the name of the rootfs, e.g. if --rootfs command line
is rootfs1=core-image-minimal partf.rootfs_dir is 'rootfs1'.
The code also fails when image name is not provided in wic
commandline. For example, when wic is called with
--rootfs-dir=<path> part.rootfs_dir will contain path and
wic will crash trying to call bitbake -e <path> to get
value of ROOTFS_SIZE variable.
Fixed the code by getting image name properly and checking
if it's not a path.
[YOCTO #10815]
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
