gemini: lazy set IMAGE_NAME

Currently, IMAGE_NAME is expanded at declaration time
and this causes strange filename in the builder's logs:

|cp: cannot stat '[...]/openwrt-gemini-dlink-dns-313-.': No such file or directory
|cp: cannot stat '[...]/openwrt-gemini-nas4220b-.': No such file or directory
|[...]

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

target/linux/gemini/image/Makefile

@@ -91,7 +91,7 @@ define Device/Default
 	KERNEL_NAME := zImage
 	KERNEL := kernel-bin | append-dtb
 	FILESYSTEMS := squashfs
-	IMAGE_NAME := $$(IMAGE_PREFIX)-$$(1).$$(2)
+	IMAGE_NAME = $$(IMAGE_PREFIX)-$$(1).$$(2)
 	BLOCKSIZE := 128k
 	PAGESIZE := 2048
 endef