boot: Modify common code access for flash_area objects

The commit changes direct access to flash_area, and flash_structure,
objects via filed names to access via getter functions
Purpose of this change is to make common code agnostic to internal
implementation of flash_area, and flash_sector, objects.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/encrypted.c b/boot/bootutil/src/encrypted.c
index 278f51d..2ea1bef 100644
--- a/boot/bootutil/src/encrypted.c
+++ b/boot/bootutil/src/encrypted.c
@@ -663,7 +663,7 @@
     uint8_t slot;
     int rc;
 
-    rc = flash_area_id_to_multi_image_slot(image_index, fap->fa_id);
+    rc = flash_area_id_to_multi_image_slot(image_index, flash_area_get_id(fap));
     if (rc < 0) {
         return rc;
     }
@@ -710,7 +710,7 @@
 {
     int rc;
 
-    rc = flash_area_id_to_multi_image_slot(image_index, fap->fa_id);
+    rc = flash_area_id_to_multi_image_slot(image_index, flash_area_get_id(fap));
     if (rc < 0) {
         /* can't get proper slot number - skip encryption, */
         /* postpone the error for a upper layer */
@@ -742,7 +742,7 @@
     nonce[14] = (uint8_t)(off >> 8);
     nonce[15] = (uint8_t)off;
 
-    rc = flash_area_id_to_multi_image_slot(image_index, fap->fa_id);
+    rc = flash_area_id_to_multi_image_slot(image_index, flash_area_get_id(fap));
     if (rc < 0) {
         assert(0);
         return;