rockchip: make_fit_atf.py: fix .its generation for a single atf image

The commit 619f002d ("rockchip: make_fit_atf.py: fix loadables property
set error") fixed the double-loading of the primary atf-image, but didn't
take into account that there may be rare atf images with only that main
section present.

Right now this will result in a broken its due to the loadables section not
getting closed correctly, so fix that by adapting the guards around the loop.

The guards now protect against 0 segments when the bl31 binary doesn't
contain any section and 1 segment when only a core atf section is present.

Fixes: 619f002d ("rockchip: make_fit_atf.py: fix loadables property set error")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>