targetcontrol.py: make possible dynamical determination of rootfs type
YB: #6375
Added a new method get_image_fstype() that autodetermines what fstype to use for the rootfs file.
This method uses a new list variable 'supported_image_fstypes' that contains image fstypes supported by the target controller.
This method is also a classmethod which means outside scripts can get the image fstype.
Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
