boot: main: avoid unused build warning

In case ESP32 SoC is used, *start will get
build warning as it is not used.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>

boot/zephyr/main.c

@@ -277,8 +277,6 @@ static void copy_img_to_SRAM(int slot, unsigned int hdr_offset)
  */
 static void do_boot(struct boot_rsp *rsp)
 {
- void *start;
-
  BOOT_LOG_INF("br_image_off = 0x%x\n", rsp->br_image_off);
  BOOT_LOG_INF("ih_hdr_size = 0x%x\n", rsp->br_hdr->ih_hdr_size);
 
@@ -292,7 +290,7 @@ static void do_boot(struct boot_rsp *rsp)
  copy_img_to_SRAM(0, rsp->br_hdr->ih_hdr_size);
 
  /* Jump to entry point */
- start = (void *)(SRAM_BASE_ADDRESS + rsp->br_hdr->ih_hdr_size);
+ void *start = (void *)(SRAM_BASE_ADDRESS + rsp->br_hdr->ih_hdr_size);
  ((void (*)(void))start)();
 #endif /* CONFIG_SOC_FAMILY_ESPRESSIF_ESP32 */
 }