Add Zephyr infrastructure for ed25519

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 3de379f..d00cbbc 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -76,6 +76,11 @@
 config BOOT_SIGNATURE_TYPE_ECDSA_P256
 	bool "Elliptic curve digital signatures with curve P-256"
 
+config BOOT_SIGNATURE_TYPE_ED25519
+	bool "Edwards curve digital signatures using ed25519"
+	select BOOT_USE_MBEDTLS
+	select MBEDTLS
+
 if BOOT_SIGNATURE_TYPE_ECDSA_P256
 choice
 	prompt "Ecdsa implementation"