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/boot/zephyr/keys.c b/boot/zephyr/keys.c
index ab403dd..f1d7e34 100644
--- a/boot/zephyr/keys.c
+++ b/boot/zephyr/keys.c
@@ -86,3 +86,12 @@
 #elif defined(MCUBOOT_ENCRYPT_KW)
 #error "Encrypted images with AES-KW is not implemented yet."
 #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 */