boot/zephyr: set VTOR to relay vector before chain-load

If software IRQ relay is enabled and VTOR is available,
the bootloader will switch from its real vector table
to the relay vector table before chain-loading to the
application.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>
Signed-off-by: Krzysztof Taborowski <krzysztof.taborowski@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>