[crypto] Add Ephemeral Diffie-Hellman key exchange algorithm

Add an implementation of the Ephemeral Diffie-Hellman key exchange
algorithm as defined in RFC2631, with test vectors taken from the NIST
Cryptographic Toolkit.

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

src/tests/tests.c

@@ -78,3 +78,4 @@ REQUIRE_OBJECT ( gzip_test );
 REQUIRE_OBJECT ( utf8_test );
 REQUIRE_OBJECT ( acpi_test );
 REQUIRE_OBJECT ( hmac_test );
+REQUIRE_OBJECT ( dhe_test );