imx6: image: fix Gateworks Ventana boot script filename

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>