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/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index c1d2a1e..7d15ce1 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -282,11 +282,11 @@
 
     if (off == 0) {
         curr_off = 0;
-        if (data_len > fap->fa_size) {
+        if (data_len > flash_area_get_size(fap)) {
             goto out_invalid_data;
         }
 #ifndef CONFIG_BOOT_ERASE_PROGRESSIVELY
-        rc = flash_area_erase(fap, 0, fap->fa_size);
+        rc = flash_area_erase(fap, 0, flash_area_get_size(fap));
         if (rc) {
             goto out_invalid_data;
         }
@@ -316,10 +316,11 @@
         BOOT_LOG_ERR("Unable to determine flash sector size");
         goto out;
     }
-    if (off_last != sector.fs_off) {
-        off_last = sector.fs_off;
-        BOOT_LOG_INF("Erasing sector at offset 0x%x", sector.fs_off);
-        rc = flash_area_erase(fap, sector.fs_off, sector.fs_size);
+    if (off_last != flash_sector_get_off(&sector)) {
+        off_last = flash_sector_get_off(&sector);
+        BOOT_LOG_INF("Erasing sector at offset 0x%x", flash_sector_get_off(&sector));
+        rc = flash_area_erase(fap, flash_sector_get_off(&sector),
+                              flash_sector_get_size(&sector));
         if (rc) {
             BOOT_LOG_ERR("Error %d while erasing sector", rc);
             goto out;
@@ -367,9 +368,11 @@
             }
             /* Assure that sector for image trailer was erased. */
             /* Check whether it was erased during previous upload. */
-            if (off_last < sector.fs_off) {
-                BOOT_LOG_INF("Erasing sector at offset 0x%x", sector.fs_off);
-                rc = flash_area_erase(fap, sector.fs_off, sector.fs_size);
+            if (off_last < flash_sector_get_off(&sector)) {
+                BOOT_LOG_INF("Erasing sector at offset 0x%x",
+                             flash_sector_get_off(&sector));
+                rc = flash_area_erase(fap, flash_sector_get_off(&sector),
+                                      flash_sector_get_size(&sector));
                 if (rc) {
                     BOOT_LOG_ERR("Error %d while erasing sector", rc);
                     goto out;