bootutil: Provide boot_set_next function

Commit provides boot_set_next function that allows to set next
application slot to boot by flash area object pointer, describing
the slot.
The function also takes active which is supposed to indicate whether
running application is being set for next boot and confirm parameter
that allows to confirm the image.

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