devicetree: auto-disable dts for old kernels

After '6392a63 rpi-base.inc: Use KERNEL_DEVICETREE by default' was
introduced, kernel versions < 3.18 might not be buildable. Since full
device tree support was introduced in 3.18 this change ensures that all
kernel < 3.18 will automatically disable device tree.

Signed-off-by: Petter Mabäcker <petter@technux.se>

classes/sdcard_image-rpi.bbclass

@@ -1,4 +1,5 @@
 inherit image_types
+inherit linux-raspberrypi-base
 
 #
 # Create an image that can by written onto a SD card using dd.
@@ -70,11 +71,6 @@ SDIMG = "${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.rpi-sdimg"
 # Additional files and/or directories to be copied into the vfat partition from the IMAGE_ROOTFS.
 FATPAYLOAD ?= ""
 
-# Device Tree Overlays are assumed to be suffixed by '-overlay.dtb' string and will be put in a dedicated folder
-DT_ALL = "${@d.getVar('KERNEL_DEVICETREE', True) or ''}"
-DT_OVERLAYS = "${@oe.utils.str_filter('\S+\-overlay\.dtb$', '${DT_ALL}', d)}"
-DT_ROOT = "${@oe.utils.str_filter_out('\S+\-overlay\.dtb$', '${DT_ALL}', d)}"
-
 IMAGEDATESTAMP = "${@time.strftime('%Y.%m.%d',time.gmtime())}"
 
 IMAGE_CMD_rpi-sdimg () {
@@ -90,6 +86,9 @@ IMAGE_CMD_rpi-sdimg () {
 
 	echo "Creating filesystem with Boot partition ${BOOT_SPACE_ALIGNED} KiB and RootFS ${ROOTFS_SIZE_ALIGNED} KiB"
 
+	# Check if we are building with device tree support
+	DTS="${@get_dts(d, None)}"
+
 	# Initialize sdcard image file
 	dd if=/dev/zero of=${SDIMG} bs=1024 count=0 seek=${SDIMG_SIZE}
 
@@ -112,7 +111,11 @@ IMAGE_CMD_rpi-sdimg () {
 		mcopy -i ${WORKDIR}/boot.img -s ${DEPLOY_DIR_IMAGE}/${KERNEL_IMAGETYPE}${KERNEL_INITRAMFS}-${MACHINE}.bin ::uImage
 		;;
 	*)
-		if test -n "${KERNEL_DEVICETREE}"; then
+		if test -n "${DTS}"; then
+			# Device Tree Overlays are assumed to be suffixed by '-overlay.dtb' string and will be put in a dedicated folder
+			DT_OVERLAYS="${@split_overlays(d, 0)}"
+			DT_ROOT="${@split_overlays(d, 1)}"
+
 			# Copy board device trees to root folder
 			for DTB in ${DT_ROOT}; do
 				DTB_BASE_NAME=`basename ${DTB} .dtb`