zephyr: arm: Update reading the flash image reset vector

This change uses the flash functions to read the applications
reset vector. This allows flexibility on where the
application image is programmed.
The current implementation for in Zephyr for ARM platforms
does not allow the use-case where the application is programmed
to a different flash device than where mcuboot is programmed
and running from.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>

boot/zephyr/main.c

@@ -178,16 +178,22 @@ static void do_boot(struct boot_rsp *rsp)
     /* Get ram address for image */
     vt = (struct arm_vector_table *)(rsp->br_hdr->ih_load_addr + rsp->br_hdr->ih_hdr_size);
 #else
-    uintptr_t flash_base;
     int rc;
+    const struct flash_area *fap;
+    int area_id;
+    uint32_t dst[2];
 
     /* Jump to flash image */
-    rc = flash_device_base(rsp->br_flash_dev_id, &flash_base);
+    area_id = flash_area_id_from_image_slot(0);
+    rc = flash_area_open(area_id, &fap);
     assert(rc == 0);
 
-    vt = (struct arm_vector_table *)(flash_base +
-                                     rsp->br_image_off +
-                                     rsp->br_hdr->ih_hdr_size);
+    rc = flash_area_read(fap, rsp->br_hdr->ih_hdr_size, dst, 8);
+    assert(rc == 0);
+
+    flash_area_close(fap);
+
+    vt = (struct arm_vector_table *)dst;
 #endif
 
     if (IS_ENABLED(CONFIG_SYSTEM_TIMER_HAS_DISABLE_SUPPORT)) {