boot: Make ft_board_setup_ex() generic

boot: Make ft_board_setup_ex() generic

In some use cases, board-specific device tree changes must not be
overwritten by system fixups. Although U-Boot provides
ft_board_setup_ex() for this purpose, it is currently only used on TI
Keystone. Make ft_board_setup_ex() to be a generic option, allowing its
use by other architectures/boards. To maintain backward compatibility,
enable it by default on TI Keystone.

Signed-off-by: João Paulo Gonçalves <joao.goncalves@toradex.com>