bootutil_public.c: add two user APIs

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

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: Ifca04d396b38c6c64581703794071f6b42e9dfbf