Mbed: Add support for EC256 encryption

Author: pennam

boot/mbed/app_enc_keys.c

@@ -66,6 +66,16 @@ const struct bootutil_key bootutil_enc_key = {
     .key = enc_priv_key,
     .len = &enc_priv_key_len,
 };
+#elif defined(MCUBOOT_ENCRYPT_EC256)
+
+extern const unsigned char enc_priv_key[];
+extern const unsigned int enc_priv_key_len;
+
+const struct bootutil_key bootutil_enc_key = {
+    .key = enc_priv_key,
+    .len = &enc_priv_key_len,
+};
+
 #elif defined(MCUBOOT_ENCRYPT_KW)
 #error "Encrypted images with AES-KW is not implemented yet."
 #endif