This converts the following to Kconfig:
   CONFIG_CMD_SPL

Note that trats does not actually use SPL, so this option can no-longer be
set.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>