build: make image target wait for initramfs

The image production rules does not have the initramfs-image
as a dependency. So, from make’s perspective initramfs
creation can run independently/in parallel with the image
generation code in the target's Makefile.

This is a problem for devices that have to use the initramfs
for the image creation and can lead to broken images.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

include/image.mk

@@ -480,7 +480,7 @@ define Device/Build/image
   ifndef IB
     $$(ROOTFS/$(1)/$(3)): $(if $(TARGET_PER_DEVICE_ROOTFS),target-dir-$$(ROOTFS_ID/$(3)))
   endif
-  $(KDIR)/tmp/$(call IMAGE_NAME,$(1),$(2)): $$(KDIR_KERNEL_IMAGE) $$(ROOTFS/$(1)/$(3))
+  $(KDIR)/tmp/$(call IMAGE_NAME,$(1),$(2)): $$(KDIR_KERNEL_IMAGE) $$(ROOTFS/$(1)/$(3)) $$(if $$(CONFIG_TARGET_ROOTFS_INITRAMFS),$(KDIR)/tmp/$$(KERNEL_INITRAMFS_IMAGE))
 	@rm -f $$@
 	[ -f $$(word 1,$$^) -a -f $$(word 2,$$^) ]
 	$$(call concat_cmd,$(if $(IMAGE/$(2)/$(1)),$(IMAGE/$(2)/$(1)),$(IMAGE/$(2))))