boot: zephyr: add ECIES-X25519 support

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index e075591..71e9a2a 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -9,6 +9,7 @@
 CONFIG_BOOT_SWAP_SAVE_ENCTLV=n
 CONFIG_BOOT_ENCRYPT_RSA=n
 CONFIG_BOOT_ENCRYPT_EC256=n
+CONFIG_BOOT_ENCRYPT_X25519=n
 
 CONFIG_BOOT_UPGRADE_ONLY=n
 CONFIG_BOOT_BOOTSTRAP=n