Assert flash ops succeed during swap.

Previously, the return of boot_swap_sectors() was discarded.  If a flash
operation failed, this information was lost.

Now, boot_swap_sectors() returns void.  Rather than returning a result,
success is asserted for all flash operations during function execution
(boot_copy_sector() and boot_erase_sector()).
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 7766d87..89b4e13 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -787,7 +787,7 @@
  *
  * @return                      0 on success; nonzero on failure.
  */
-static int
+static void
 boot_swap_sectors(int idx, uint32_t sz, struct boot_status *bs)
 {
     uint32_t copy_sz;
@@ -800,24 +800,19 @@
 
     if (bs->state == 0) {
         rc = boot_erase_sector(FLASH_AREA_IMAGE_SCRATCH, 0, sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         rc = boot_copy_sector(FLASH_AREA_IMAGE_1, FLASH_AREA_IMAGE_SCRATCH,
                               img_off, 0, sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         bs->state = 1;
-        (void)boot_write_status(bs);
+        rc = boot_write_status(bs);
+        assert(rc == 0);
     }
     if (bs->state == 1) {
         rc = boot_erase_sector(FLASH_AREA_IMAGE_1, img_off, sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         copy_sz = sz;
         if (boot_data.imgs[0].sectors[idx].fa_off + sz >=
@@ -831,31 +826,25 @@
 
         rc = boot_copy_sector(FLASH_AREA_IMAGE_0, FLASH_AREA_IMAGE_1,
                               img_off, img_off, copy_sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         bs->state = 2;
-        (void)boot_write_status(bs);
+        rc = boot_write_status(bs);
+        assert(rc == 0);
     }
     if (bs->state == 2) {
         rc = boot_erase_sector(FLASH_AREA_IMAGE_0, img_off, sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         rc = boot_copy_sector(FLASH_AREA_IMAGE_SCRATCH, FLASH_AREA_IMAGE_0,
                               0, img_off, sz);
-        if (rc != 0) {
-            return rc;
-        }
+        assert(rc == 0);
 
         bs->idx++;
         bs->state = 0;
-        (void)boot_write_status(bs);
+        rc = boot_write_status(bs);
+        assert(rc == 0);
     }
-
-    return 0;
 }
 
 /**