fngsystem: Load fdt file and overlays directly from fitImage

This way it's the same procedure as for normal Yocto image (picked from boot_conf environment variable).

Now we can remove the separate dtb and dtbo files from the fngsystem image.

conf/machine/include/mediatek-common.inc

@@ -26,8 +26,6 @@ OPTEE_TZDRAM_SIZE  ?= "0x00a00000"
 IMAGE_BOOT_FILES = " \
 	${@bb.utils.contains('KERNEL_IMAGETYPE', 'fitImage', '', os.path.basename("${KERNEL_DEVICETREE}"), d)} \
 	${KERNEL_IMAGETYPE} \
-	${@' '.join([dtb.split('/')[-1] for dtb in d.getVar('KERNEL_DEVICETREE').split()])} \
-	${@' '.join(['devicetree/' + dtbo[:-5] + '.dtbo' for dtbo in d.getVar('KERNEL_DTBO').split()])} \
 	fngsystem-image.cpio.gz \
 "