kernel: fitimage: support device tree compiler options

This introduces a new variable to set the device tree compiler options while
calling mkimage ('-D' option). By default, this variable is not set but it can
be defined in a configuration file, as following example:

UBOOT_MKIMAGE_DTCOPTS = "-I dts -O dtb -p 2000"

(From OE-Core rev: 9a3f541da305c75418c2eec75ade902717a01b69)

Signed-off-by: Yannick Gicquel <yannick.gicquel@iot.bzh>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel-fitimage.bbclass

@@ -17,6 +17,9 @@ python __anonymous () {
             d.appendVarFlag('do_assemble_fitimage', 'depends', ' ${INITRAMFS_IMAGE}:do_image_complete')
 }
 
+# Options for the device tree compiler passed to mkimage '-D' feature:
+UBOOT_MKIMAGE_DTCOPTS ??= ""
+
 #
 # Emit the fitImage ITS header
 #
@@ -209,7 +212,10 @@ do_assemble_fitimage() {
 		#
 		# Step 4: Assemble the image
 		#
-		uboot-mkimage -f fit-image.its arch/${ARCH}/boot/fitImage
+		uboot-mkimage \
+			${@'-D "${UBOOT_MKIMAGE_DTCOPTS}"' if len('${UBOOT_MKIMAGE_DTCOPTS}') else ''} \
+			-f fit-image.its \
+			arch/${ARCH}/boot/fitImage
 	fi
 }