kernel-fitimage.bbclass: only package unique DTBs

The KERNEL_DEVICETREE and related variables could potentially have a device
tree listed multiple times and this works okay for most scenarios. However,
when we create FIT entries for these we get duplicate nodes and uboot-mkimage
fails with

fit-image-initramfs-image.its:219.58-229.19: ERROR (duplicate_node_names): /images/fdt-freescale_imx8mp-evk-ecspi-slave.dtb: Duplicate node name
fit-image-initramfs-image.its:307.50-317.19: ERROR (duplicate_node_names): /images/fdt-freescale_imx8mp-evk-ndm.dtb: Duplicate node name
fit-image-initramfs-image.its:362.54-372.19: ERROR (duplicate_node_names): /images/fdt-freescale_imx8mp-evk-rm67199.dtb: Duplicate node name
fit-image-initramfs-image.its:417.56-427.19: ERROR (duplicate_node_names): /images/fdt-freescale_imx8mp-evk-usdhc1-m2.dtb: Duplicate node name
fit-image-initramfs-image.its:648.59-658.19: ERROR (duplicate_node_names): /configurations/conf-freescale_imx8mp-evk-ecspi-slave.dtb: Duplicate node name
fit-image-initramfs-image.its:744.51-754.19: ERROR (duplicate_node_names): /configurations/conf-freescale_imx8mp-evk-ndm.dtb: Duplicate node name
fit-image-initramfs-image.its:804.55-814.19: ERROR (duplicate_node_names): /configurations/conf-freescale_imx8mp-evk-rm67199.dtb: Duplicate node name
fit-image-initramfs-image.its:864.57-874.19: ERROR (duplicate_node_names): /configurations/conf-freescale_imx8mp-evk-usdhc1-m2.dtb: Duplicate node name
ERROR: Input tree has errors, aborting (use -f to force output)
uboot-mkimage: Can't open arch/arm64/boot/fitImage.tmp: No such file or directory

We fix this by tracking the DTBs we're compiling in the FIT and only picking
up unique ones.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>