boot/zephyr: Fix Direct-XIP boot messages

The commit changes banner message to indicate that bootloader has been
built for Direct-XIP mode.
It also removes, in Direct-XIP compiled code, information on jumping
into first slot: that information is not always correct for Direct-XIP
and other messages correctly point slot that will be used.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>