[tls] Make key exchange algorithms selectable via build configuration

Allow the choice of key exchange algorithms to be controlled via build
configuration options in config/crypto.h, as is already done for the
choices of public-key algorithms, cipher algorithms, and digest
algorithms.

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

src/config/crypto.h

@@ -12,6 +12,12 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /** Minimum TLS version */
 #define TLS_VERSION_MIN TLS_VERSION_TLS_1_1
 
+/** Public-key exchange algorithm */
+#define CRYPTO_EXCHANGE_PUBKEY
+
+/** DHE key exchange algorithm */
+#define CRYPTO_EXCHANGE_DHE
+
 /** RSA public-key algorithm */
 #define CRYPTO_PUBKEY_RSA