[nrf fromtree] boot/bootutil/src/loader: fix Cortex reset-vector offset

While reading cortex Vector table, the reset handler addres is
at offest of one 4 bytes word, not two words.

This patch corrects this offset.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
(cherry picked from commit f48e87edd473abdc966c03fadb8a00305e466732)

Author: nvlsianpu

boot/bootutil/src/loader.c

@@ -147,7 +147,7 @@ static const struct image_version mcuboot_s0_s1_image_version = {
 #endif
 
 /* Valid only for ARM Cortext M */
-#define RESET_OFFSET    (2 * sizeof(uint32_t))
+#define RESET_OFFSET sizeof(uint32_t)
 
 #if BOOT_MAX_ALIGN > 1024
 #define BUF_SZ BOOT_MAX_ALIGN