kirkwood: clean up redundant variables in the image building code

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/kirkwood/image/Makefile

@@ -23,18 +23,20 @@ define Device/Default
   KERNEL_NAME := zImage
   KERNEL_SUFFIX  := -uImage
   KERNEL_INSTALL := 1
-endef
 
-define Device/dockstar
-  DEVICE_DTS := kirkwood-dockstar
   PAGESIZE := 2048
   SUBPAGESIZE := 512
   BLOCKSIZE := 128KiB
+  IMAGES := sysupgrade.tar
+  IMAGE/sysupgrade.tar := sysupgrade-nand
+endef
+
+define Device/dockstar
+  DEVICE_DTS := kirkwood-dockstar
   FILESYSTEMS := squashfs
   PROFILES := Generic DOCKSTAR
-  IMAGES := factory.bin sysupgrade.tar
+  IMAGES += factory.bin
   IMAGE/factory.bin := append-ubi
-  IMAGE/sysupgrade.tar := sysupgrade-nand
   KERNEL_IN_UBI := 1
   KERNEL := kernel-bin | append-dtb
 endef
@@ -53,28 +55,20 @@ endef
 
 define Device/linksys-audi
   DEVICE_DTS := kirkwood-linksys-audi
-  PAGESIZE := 512
-  SUBPAGESIZE := 256
-  BLOCKSIZE := 16KiB
   KERNEL_SIZE := 2624k
   FILESYSTEMS := squashfs
   PROFILES := Generic AUDI
-  IMAGES := factory.bin sysupgrade.tar
+  IMAGES += factory.bin
   IMAGE/factory.bin := append-kernel $$$$(KERNEL_SIZE) | append-ubi
-  IMAGE/sysupgrade.tar := sysupgrade-nand
 endef
 
 define Device/linksys-viper
   DEVICE_DTS := kirkwood-linksys-viper
-  PAGESIZE := 2048
-  SUBPAGESIZE := 512
-  BLOCKSIZE := 128KiB
   KERNEL_SIZE := 2688k
   FILESYSTEMS := squashfs
   PROFILES := Generic VIPER
-  IMAGES := factory.bin sysupgrade.tar
+  IMAGES += factory.bin
   IMAGE/factory.bin := append-kernel $$$$(KERNEL_SIZE) | append-ubi
-  IMAGE/sysupgrade.tar := sysupgrade-nand
 endef
 
 define Image/BuildKernel/Template