boot/zephyr: switch main return type to 'int'

Adapt to Zephyr's change requiring main to return int.

Signed-off-by: Keith Packard <keithp@keithp.com>

boot/zephyr/main.c

@@ -530,7 +530,7 @@ static void boot_serial_enter()
 }
 #endif
 
-void main(void)
+int main(void)
 {
     struct boot_rsp rsp;
     int rc;