boot: boot_serial: Fix mynewt build without encryption

boot_serial_encryption.c can exclude all code when
MCUBOOT_ENC_IMAGES is not defined.

However it first includes several headers that fail
to build in mynewt where MCUBOOT_ENC_IMAGES is always
undefined (so far).

This change includes configuration first and if
MCUBOOT_ENC_IMAGES is not defined it also skips
unnecessary includesion of headers and not only
actual code for encrypted images over serial.

Error that was detected:
repos/mcuboot/boot/bootutil/include/bootutil/crypto/aes_ctr.h: In function 'bootutil_aes_ctr_encrypt':
repos/mcuboot/boot/bootutil/include/bootutil/crypto/aes_ctr.h:100:12: error: implicit declaration of function 'mbedtls_aes_crypt_ctr'; did you mean 'mbedtls_aes_crypt_ecb'? [-Wimplicit-function-declaration]
  100 |     return mbedtls_aes_crypt_ctr(ctx, mlen, &blk_off, counter, stream_block, m, c);
      |            ^~~~~~~~~~~~~~~~~~~~~
      |            mbedtls_aes_crypt_ecb

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>

Author: kasjer

boot/boot_serial/src/boot_serial_encryption.c

@@ -5,17 +5,17 @@
  * Copyright (c) 2020 Arm Limited
  */
 
+#include "mcuboot_config/mcuboot_config.h"
+
+#ifdef MCUBOOT_ENC_IMAGES
+
 #include <assert.h>
 #include "bootutil/image.h"
 #include <../src/bootutil_priv.h>
 #include "bootutil/bootutil_log.h"
 #include "bootutil/bootutil_public.h"
 #include "bootutil/fault_injection_hardening.h"
 
-#include "mcuboot_config/mcuboot_config.h"
-
-#ifdef MCUBOOT_ENC_IMAGES
-
 BOOT_LOG_MODULE_DECLARE(serial_encryption);
 
 fih_ret