board: Add a gd flag for chain loading

When U-Boot is run from another boot loader, much of the low-level init
needs to be skipped.

Add a flag for this and adjust ll_boot_init() to use it.

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