boot: Add MCUBOOT_HW_KEY support for image encryption

Currently encryption supports only private key embed
in mcuboot itself. To support MCUBOOT_HW_KEY for image
encryption boot_retrieve_private_key() hook is added.

This hook helps retrieving private key from trusted
sources like OTP, TPM.

Signed-off-by: Dinesh Kumar K <dinesh@linumiz.com>
diff --git a/sim/mcuboot-sys/csupport/keys.c b/sim/mcuboot-sys/csupport/keys.c
index 82a746b..316d103 100644
--- a/sim/mcuboot-sys/csupport/keys.c
+++ b/sim/mcuboot-sys/csupport/keys.c
@@ -328,3 +328,12 @@
     .len = &enc_key_len,
 };
 #endif
+
+#if !defined(MCUBOOT_HW_KEY) && defined(MCUBOOT_ENC_IMAGES)
+int boot_enc_retrieve_private_key(struct bootutil_key **private_key)
+{
+    *private_key = (struct bootutil_key *)&bootutil_enc_key;
+
+    return 0;
+}
+#endif /* !MCUBOOT_HW_KEY && MCUBOOT_ENC_IMAGES */