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/swap_misc.c b/boot/bootutil/src/swap_misc.c
index 940d646..343e9fe 100644
--- a/boot/bootutil/src/swap_misc.c
+++ b/boot/bootutil/src/swap_misc.c
@@ -46,7 +46,7 @@
     uint8_t image_index;
     int rc;
 
-    BOOT_LOG_DBG("erasing trailer; fa_id=%d", fap->fa_id);
+    BOOT_LOG_DBG("erasing trailer; fa_id=%d", flash_area_get_id(fap));
 
     image_index = BOOT_CURR_IMG(state);
     fa_id_primary = flash_area_id_from_multi_image_slot(image_index,
@@ -54,9 +54,9 @@
     fa_id_secondary = flash_area_id_from_multi_image_slot(image_index,
             BOOT_SECONDARY_SLOT);
 
-    if (fap->fa_id == fa_id_primary) {
+    if (flash_area_get_id(fap) == fa_id_primary) {
         slot = BOOT_PRIMARY_SLOT;
-    } else if (fap->fa_id == fa_id_secondary) {
+    } else if (flash_area_get_id(fap) == fa_id_secondary) {
         slot = BOOT_SECONDARY_SLOT;
     } else {
         return BOOT_EFLASH;
@@ -94,7 +94,7 @@
 
     image_index = BOOT_CURR_IMG(state);
 
-    BOOT_LOG_DBG("initializing status; fa_id=%d", fap->fa_id);
+    BOOT_LOG_DBG("initializing status; fa_id=%d", flash_area_get_id(fap));
 
     rc = boot_read_swap_state_by_id(FLASH_AREA_IMAGE_SECONDARY(image_index),
             &swap_state);