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/bootutil_misc.c b/boot/bootutil/src/bootutil_misc.c
index 9ee317f..e8cef25 100644
--- a/boot/bootutil/src/bootutil_misc.c
+++ b/boot/bootutil/src/bootutil_misc.c
@@ -123,12 +123,12 @@
 boot_status_entries(int image_index, const struct flash_area *fap)
 {
 #if MCUBOOT_SWAP_USING_SCRATCH
-    if (fap->fa_id == FLASH_AREA_IMAGE_SCRATCH) {
+    if (flash_area_get_id(fap) == FLASH_AREA_IMAGE_SCRATCH) {
         return BOOT_STATUS_STATE_COUNT;
     } else
 #endif
-    if (fap->fa_id == FLASH_AREA_IMAGE_PRIMARY(image_index) ||
-               fap->fa_id == FLASH_AREA_IMAGE_SECONDARY(image_index)) {
+    if (flash_area_get_id(fap) == FLASH_AREA_IMAGE_PRIMARY(image_index) ||
+        flash_area_get_id(fap) == FLASH_AREA_IMAGE_SECONDARY(image_index)) {
         return BOOT_STATUS_STATE_COUNT * BOOT_STATUS_MAX_ENTRIES;
     }
     return -1;
@@ -144,14 +144,14 @@
 
     off_from_end = boot_trailer_sz(elem_sz);
 
-    assert(off_from_end <= fap->fa_size);
-    return fap->fa_size - off_from_end;
+    assert(off_from_end <= flash_area_get_size(fap));
+    return flash_area_get_size(fap) - off_from_end;
 }
 
 static inline uint32_t
 boot_magic_off(const struct flash_area *fap)
 {
-    return fap->fa_size - BOOT_MAGIC_SZ;
+    return flash_area_get_size(fap) - BOOT_MAGIC_SZ;
 }
 
 static inline uint32_t
@@ -301,8 +301,8 @@
 
     off = boot_copy_done_off(fap);
     BOOT_LOG_DBG("writing copy_done; fa_id=%d off=0x%lx (0x%lx)",
-                 fap->fa_id, (unsigned long)off,
-                 (unsigned long)(fap->fa_off + off));
+                 flash_area_get_id(fap), (unsigned long)off,
+                 (unsigned long)(flash_area_get_off(fap) + off));
     return boot_write_trailer_flag(fap, off, BOOT_FLAG_SET);
 }
 
@@ -313,8 +313,8 @@
 
     off = boot_swap_size_off(fap);
     BOOT_LOG_DBG("writing swap_size; fa_id=%d off=0x%lx (0x%lx)",
-                 fap->fa_id, (unsigned long)off,
-                 (unsigned long)fap->fa_off + off);
+                 flash_area_get_id(fap), (unsigned long)off,
+                 (unsigned long)flash_area_get_off(fap) + off);
     return boot_write_trailer(fap, off, (const uint8_t *) &swap_size, 4);
 }
 
@@ -328,8 +328,8 @@
 
     off = boot_enc_key_off(fap, slot);
     BOOT_LOG_DBG("writing enc_key; fa_id=%d off=0x%lx (0x%lx)",
-                 fap->fa_id, (unsigned long)off,
-                 (unsigned long)fap->fa_off + off);
+                 flash_area_get_id(fap), (unsigned long)off,
+                 (unsigned long)flash_area_get_off(fap) + off);
 #if MCUBOOT_SWAP_SAVE_ENCTLV
     rc = flash_area_write(fap, off, bs->enctlv[slot], BOOT_ENC_TLV_ALIGN_SIZE);
 #else