zephyr: Switch single loader to use flash map getters

The commit changes single loader code that has been accessing
flash_area type object directly, to use the new getter functions.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/single_loader.c b/boot/zephyr/single_loader.c
index c16a0b7..9b24f3e 100644
--- a/boot/zephyr/single_loader.c
+++ b/boot/zephyr/single_loader.c
@@ -83,7 +83,7 @@
     }
 
     if (!boot_u32_safe_add(&size, hdr->ih_img_size, hdr->ih_hdr_size) ||
-        size >= fa_p->fa_size) {
+        size >= flash_area_get_size(fa_p)) {
         return BOOT_EBADIMAGE;
     }
 
@@ -120,8 +120,8 @@
     fih_rc = FIH_SUCCESS;
 #endif /* MCUBOOT_VALIDATE_PRIMARY_SLOT */
 
-    rsp->br_flash_dev_id = _fa_p->fa_device_id;
-    rsp->br_image_off = _fa_p->fa_off;
+    rsp->br_flash_dev_id = flash_area_get_device_id(_fa_p);
+    rsp->br_image_off = flash_area_get_off(_fa_p);
     rsp->br_hdr = &_hdr;
 
 out: