In order to not define a CONFIG value when the
CONFIG_AUTOBOOT_STOP_STR_* functionality is not enabled, rework the
assignment of empty and unused (as the code will be discarded under if
0, in the end) values to be AUTOBOOT_STOP_STR_* instead of
CONFIG_AUTOBOOT_STOP_STR_*.

Signed-off-by: Tom Rini <trini@konsulko.com>