gemini: Name binary "bootpart.tar.gz"

This factory binary i supposed to actually be unzipped and
untarred by the user as part of the installation process
(this NAS boots from harddisk), so name it "bootpart.tar.gz"
and not "factory.bin" so it is helpful for users.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/gemini/image/Makefile

@@ -123,8 +123,8 @@ define Device/dlink_dns-313
 	DEVICE_TITLE := D-Link DNS-313 1-Bay Network Storage Enclosure
 	DEVICE_DTS := gemini-dlink-dns-313
 	DEVICE_PACKAGES := $(GEMINI_NAS_PACKAGES)
-	IMAGES := factory.bin
-	IMAGE/factory.bin := dns313-images
+	IMAGES := bootpart.tar.gz
+	IMAGE/bootpart.tar.gz := dns313-images
 endef
 TARGET_DEVICES += dlink_dns-313