toasterui: do not filter images by extension

toasterui: do not filter images by extension

ToasterUI filters build artifacts by extension in order
to determine if a build artifact is an image or not.

Using IMAGE_FSTYPES for this purpose is not correct as
the varible value holding image extensions is just a coincidence.

So we just look if the filename contains the "rootfs" magic
string, which is a pretty good approximation.

[YOCTO #7213]

Signed-off-by: Alexandru Damian <alexandru.damian@intel.com>