bootutil: Fix boot_set_next passing wrong image number

The commit fixes boot_set_next always passing image 0 to
boot_write_swap_info, instead of the proper image number.
This has been affecting applications that tried to call boot_set_next
in multi-image MCUboot configuration using scratch-swap algorithm.

Fixes #1762

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