zephyr: Add a bool Kconfig option for swap move

The Kconfig system used by Zephyr does not allow the defaults for choice
options to be overridden.  To compensate for this, create a new boolean
config option that will determine what the default is for the boot mode.

This allows the kconfig override file for various Zephyr boards to
change the default to swap move.

Signed-off-by: David Brown <david.brown@linaro.org>