boot: Change boot_enc_load to take slot number instead of image

In all cases where boot_enc_load is called it is known what slot
is addressed, so it is better to just pass the slot number
instead of making the boot_enc_load figure out slot number from
image index and provided flash area object.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 7f9ac979)