env: ti: ti_common.env: Fix get_overlaystring for FIT Image
After the refactor with conf- nodes in fitImage, overlaystring wasn't
didn't handle the new conf- nodes in FIT Booting. Fix get_overlaystring
to handle conf- nodes.
Fixes: 837833a724b7 ("environment: ti: Add get_fit_config command to get FIT config string") Reported-by: Aniket Limaye <a-limaye@ti.com> Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com> Reviewed-by: Andrew Davis <afd@ti.com>