kernel-fitimage: skip FDT section creation for applicable symlinks

When building a FIT image with device trees, each device tree lands in a
FIT section and is referenced by a FIT configuration node.

FIT images however also allow referencing the same device tree from
multiple configurations. This can be useful to reduce FIT image size
while staying compatible with existing bootloaders. Allow
kernel-fitimage.bbclass users to take advantage of this by mapping
each symlink to a regular device tree included in the FIT to a
configuration that references a common device tree section.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>