caps: Add cap for x25519 based image encryption

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index c15ecb5..a59a01d 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -54,6 +54,9 @@
 #if defined(MCUBOOT_ENCRYPT_EC256)
     res |= BOOTUTIL_CAP_ENC_EC256;
 #endif
+#if defined(MCUBOOT_ENCRYPT_X25519)
+    res |= BOOTUTIL_CAP_ENC_X25519;
+#endif
 #if defined(MCUBOOT_VALIDATE_PRIMARY_SLOT)
     res |= BOOTUTIL_CAP_VALIDATE_PRIMARY_SLOT;
 #endif