boot/zephyr: feed watchdog at the start

Added call to the watchdog feed macro at the
MCUBoot's main() beginning.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 350b3ab..0082069 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -313,6 +313,8 @@
     int rc;
     fih_int fih_rc = FIH_FAILURE;
 
+    MCUBOOT_WATCHDOG_FEED();
+
     BOOT_LOG_INF("Starting bootloader");
 
     os_heap_init();