imx6: image: drop BOOT_SCRIPT and fix DEVICE_NAME

imx6: image: drop BOOT_SCRIPT and fix DEVICE_NAME

This fixes Gateworks Ventana 'DEVICE_NAME' variable which value wasn't
adjusted during migration to common 'vendor_model' image naming scheme
(fixes: FS#3825).

Furthermore, drop unused 'Build/boot-scr' recipe, get rid of redundant
'BOOT_SCRIPT' variable (use already provided 'DEVICE_NAME' instead) and
drop custom 'DEVICE_NAME' variable from SolidRun CuBox-i image recipe
(use default one instead).

Fixes: cbc8bcfbaa ("imx6: image: use vendor_model scheme")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>