include/image*: add support for device-tree overlays

Add new target feature 'dt-overlay' which makes DTC keep the symbol
names in the generated dtb.
Make sure additional DT overlay sources specified by the new device
variable DEVICE_DTS_OVERLAY get compiled together with the main DTS
(currently overlays got to be in the same folder). Let Build/fit pass
the generated DT overlay blobs to mkits.sh.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

include/image.mk

@@ -190,6 +190,7 @@ define Image/BuildDTB
 		-o $(2).tmp $(1)
 	$(LINUX_DIR)/scripts/dtc/dtc -O dtb \
 		-i$(dir $(1)) $(DTC_FLAGS) $(4) \
+	$(if $(CONFIG_HAS_DT_OVERLAY_SUPPORT),-@) \
 		-o $(2) $(2).tmp
 	$(RM) $(2).tmp
 endef
@@ -393,6 +394,7 @@ define Device/Init
   DEVICE_DTS :=
   DEVICE_DTS_CONFIG :=
   DEVICE_DTS_DIR :=
+  DEVICE_DTS_OVERLAY :=
   DEVICE_FDT_NUM :=
   SOC :=
 
@@ -416,8 +418,8 @@ DEFAULT_DEVICE_VARS := \
   DEVICE_NAME KERNEL KERNEL_INITRAMFS KERNEL_INITRAMFS_IMAGE KERNEL_SIZE \
   CMDLINE UBOOTENV_IN_UBI KERNEL_IN_UBI BLOCKSIZE PAGESIZE SUBPAGESIZE \
   VID_HDR_OFFSET UBINIZE_OPTS UBINIZE_PARTS MKUBIFS_OPTS DEVICE_DTS \
-  DEVICE_DTS_CONFIG DEVICE_DTS_DIR DEVICE_FDT_NUM SOC BOARD_NAME \
-  UIMAGE_MAGIC UIMAGE_NAME \
+  DEVICE_DTS_CONFIG DEVICE_DTS_DIR DEVICE_DTS_OVERLAY DEVICE_FDT_NUM \
+  SOC BOARD_NAME UIMAGE_MAGIC UIMAGE_NAME \
   SUPPORTED_DEVICES IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR \
   UBOOT_PATH IMAGE_SIZE \
   DEVICE_COMPAT_VERSION DEVICE_COMPAT_MESSAGE \
@@ -539,7 +541,7 @@ endef
 endif
 
 define Device/Build/kernel
-  $$(eval $$(foreach dts,$$(DEVICE_DTS), \
+  $$(eval $$(foreach dts,$$(DEVICE_DTS) $$(DEVICE_DTS_OVERLAY), \
 	$$(call Device/Build/dtb,$$(notdir $$(dts)), \
 		$$(if $$(DEVICE_DTS_DIR),$$(DEVICE_DTS_DIR),$$(DTS_DIR)), \
 		$$(dts) \