boot/zephyr/main: fix placement of pointer to arm vector

pointer to the image ARM vector table should be placed out of
stack which is being reconfigured before vt is used for branch
to the application. This caused transient boot failure when
CONFIG_LTO=y.

Moved vt to static data scope.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>