[crypto] Generalise elliptic curve key exchange to ecdhe_key()

Split out the portion of tls_send_client_key_exchange_ecdhe() that
actually performs the elliptic curve key exchange into a separate
function ecdhe_key().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/ecdhe.h

@@ -0,0 +1,17 @@
+#ifndef _IPXE_ECDHE_H
+#define _IPXE_ECDHE_H
+
+/** @file
+ *
+ * Elliptic Curve Ephemeral Diffie-Hellman (ECDHE) key exchange
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/crypto.h>
+
+extern int ecdhe_key ( struct elliptic_curve *curve, const void *partner,
+		       const void *private, void *public, void *shared );
+
+#endif /* _IPXE_ECDHE_H */