dfu/boot: request upgrade & confirm for multimage

Add boot_write_confirm_multi() and boot_request_upgrade_multi() APIs so
that the user can set the image with given index as pending, confirmed.

This is needed for enabling the mcuboot multi-mage boot feature by
the zephyr.

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