build: fix build for devices without initramfs

Don't attempt to copy initramfs images for devices which do not output
an initramfs image.

This was breaking builds for mpc85xx-p1010 since mid-march.

Signed-off-by: David Bauer <mail@david-bauer.net>

include/image.mk

@@ -478,7 +478,7 @@ define Device/Build/initramfs
 	  $$(if $$(CONFIG_JSON_OVERVIEW_IMAGE_INFO), $(BUILD_DIR)/json_info_files/$$(KERNEL_INITRAMFS_IMAGE).json,))
 
   $(KDIR)/$$(KERNEL_INITRAMFS_NAME):: image_prepare
-  $(1)-images: $(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE)
+  $(1)-images: $$(if $$(KERNEL_INITRAMFS),$(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE))
   $(BIN_DIR)/$$(KERNEL_INITRAMFS_IMAGE): $(KDIR)/tmp/$$(KERNEL_INITRAMFS_IMAGE)
 	cp $$^ $$@