imx6: image: fix Gateworks Ventana boot script filename

U-Boot for the Gateworks Ventana includes filename of the boot script in
the default/embedded environment (see 'include/configs/gw_ventana.h' in
the U-Boot sources).

This restores the old boot script filename ('6x_bootscript-ventana'),
making Ventana boards boot again.

Fixes: 8dba71dd33 ("imx6: image: drop BOOT_SCRIPT and fix DEVICE_NAME")
Reported-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

target/linux/imx6/image/Makefile

@@ -124,7 +124,7 @@ define Device/gateworks_ventana
   DEVICE_VENDOR := Gateworks
   DEVICE_MODEL := Ventana family
   DEVICE_VARIANT := normal NAND flash
-  DEVICE_NAME := gateworks_ventana
+  DEVICE_NAME := ventana
   DEVICE_DTS:= \
 	imx6dl-gw51xx \
 	imx6dl-gw52xx \