uboot-imx

sandbox: Convert SANDBOX_BITS_PER_LONG to Kconfig

Convert SANDBOX_BITS_PER_LONG to Kconfig and assign it a correct
number depending on which host we are going to build and run.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>