Add Zephyr infrastructure for ed25519

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/include/mcuboot-mbedtls-cfg.h b/boot/zephyr/include/mcuboot-mbedtls-cfg.h
index 00f90e8..b15d5ac 100644
--- a/boot/zephyr/include/mcuboot-mbedtls-cfg.h
+++ b/boot/zephyr/include/mcuboot-mbedtls-cfg.h
@@ -25,6 +25,8 @@
 #include "config-rsa.h"
 #elif defined(CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256)
 #include "config-asn1.h"
+#elif defined(CONFIG_BOOT_SIGNATURE_TYPE_ED25519)
+#include "config-ed25519.h"
 #else
 #error "Cannot configure mbedTLS; signature type is unknown."
 #endif