[eap] Add support for the MD5-Challenge authentication type

RFC 3748 states that support for MD5-Challenge is mandatory for EAP
implementations.  The MD5 and CHAP code is already included in the
default build since it is required by iSCSI, and so this does not
substantially increase the binary size.

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

src/include/ipxe/eap.h

@@ -49,6 +49,17 @@ struct eap_message {
 /** EAP NAK */
 #define EAP_TYPE_NAK 3
 
+/** EAP MD5 challenge request/response */
+#define EAP_TYPE_MD5 4
+
+/** EAP MD5 challenge request/response type data */
+struct eap_md5 {
+	/** Value length */
+	uint8_t len;
+	/** Value */
+	uint8_t value[0];
+} __attribute__ (( packed ));
+
 /** EAP success */
 #define EAP_CODE_SUCCESS 3