autoboot: Use CONFIG_AUTOBOOT_STOP_STR_SHA256 indirectly

This CONFIG option is only present if CONFIG_AUTOBOOT_ENCRYPTION is
enabled so it cannot be used in code without that #ifdef. But we want to
reduce the use of #ifdef in this file and in particular to avoid having
two different functions both named passwd_abort() but which do different
things.

In preparation for this, create an intermediate value which is set to an
empty string if there is no value for CONFIG_AUTOBOOT_STOP_STR_SHA256.

Signed-off-by: Simon Glass <sjg@chromium.org>