Drop x25519, use p256 only

src/tls.h

@@ -90,7 +90,7 @@ void mg_tls_ctx_free(struct mg_mgr *);
 #if MG_TLS == MG_TLS_BUILTIN
 // Extracts a 32-byte P-256 private scalar from EC PRIVATE KEY or PKCS#8 PRIVATE
 // KEY input. Accepts PEM or DER. Returns key size, or 0 on error.
-size_t mg_uecc_parse_private_key(struct mg_str key, uint8_t *buf, size_t len);
+size_t mg_p256_parse_private_key(struct mg_str key, uint8_t *buf, size_t len);
 #endif
 
 // Low-level IO primitives used by TLS layer