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>
diff --git a/boot/bootutil/src/encrypted.c b/boot/bootutil/src/encrypted.c
index 09b9a78..f3516ac 100644
--- a/boot/bootutil/src/encrypted.c
+++ b/boot/bootutil/src/encrypted.c
@@ -607,7 +607,7 @@
  * Load encryption key.
  */
 int
-boot_enc_load(struct enc_key_data *enc_state, int image_index,
+boot_enc_load(struct enc_key_data *enc_state, int slot,
         const struct image_header *hdr, const struct flash_area *fap,
         struct boot_status *bs)
 {
@@ -619,15 +619,8 @@
 #else
     uint8_t buf[EXPECTED_ENC_LEN];
 #endif
-    uint8_t slot;
     int rc;
 
-    rc = flash_area_id_to_multi_image_slot(image_index, flash_area_get_id(fap));
-    if (rc < 0) {
-        return rc;
-    }
-    slot = rc;
-
     /* Already loaded... */
     if (enc_state[slot].valid) {
         return 1;