ARM: sunxi: get script.bin file name from env

All the other files can be overridden by env settings, what's so
special about script.bin?

Use the "script" env variable instead of hard-coding script.bin
everywhere; still defaults to "script.bin", but can be overridden
from uEnv.txt or boot.scr.

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>