imx6: image: ventana: Add missing BOOT_SCRIPT variable

This is probably theoretical problem as the Ventana is defined first in
the image Makefile, but once the position of the definition would change
in the future (alphabetical sorting?) it would get bootscript from the
previous board which would have BOOT_SCRIPT set.

Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/imx6/image/Makefile

@@ -136,6 +136,7 @@ define Device/ventana
   IMAGES := nand.ubi bootfs.tar.gz
   IMAGE_NAME = $$(IMAGE_PREFIX)-$$(1).$$(2)
   UBINIZE_PARTS = boot=$$(KDIR_KERNEL_IMAGE).boot.ubifs=15
+  BOOT_SCRIPT := bootscript-ventana
   IMAGE/nand.ubi := append-ubi
   IMAGE/bootfs.tar.gz := bootfs.tar.gz | install-dtb
   IMAGE_NAME = $$(IMAGE_PREFIX)-$$(1)-$$(2)