boot: zephyr: add ECIES-X25519 support

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/keys.c b/boot/zephyr/keys.c
index d5aeba4..5dd0ca2 100644
--- a/boot/zephyr/keys.c
+++ b/boot/zephyr/keys.c
@@ -201,6 +201,18 @@
     .key = enc_priv_key,
     .len = &enc_priv_key_len,
 };
+#elif defined(MCUBOOT_ENCRYPT_X25519)
+unsigned char enc_key[] = {
+  0x30, 0x2e, 0x02, 0x01, 0x00, 0x30, 0x05, 0x06, 0x03, 0x2b, 0x65, 0x6e,
+  0x04, 0x22, 0x04, 0x20, 0x28, 0x80, 0x2f, 0xef, 0xef, 0x82, 0x95, 0x50,
+  0xf1, 0x41, 0x93, 0x03, 0x6c, 0x1b, 0xb9, 0x49, 0x6c, 0x51, 0xe5, 0x26,
+  0x87, 0x8f, 0x77, 0x07, 0xf8, 0xb4, 0x1f, 0x04, 0x45, 0x6d, 0x84, 0x4f,
+};
+static unsigned int enc_key_len = 48;
+const struct bootutil_key bootutil_enc_key = {
+    .key = enc_key,
+    .len = &enc_key_len,
+};
 #elif defined(MCUBOOT_ENCRYPT_KW)
 #error "Encrypted images with AES-KW is not implemented yet."
 #endif