[tftp] Eliminate unnecessary variable-length stack allocation

Eliminate an unnecessary variable-length stack allocation and memory
copy by allowing TFTP option processors to modify the option string
in-place.

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

src/net/udp/tftp.c

  * @v value		Option value
  * @ret rc		Return status code
  */
-static int tftp_process_blksize ( struct tftp_request *tftp,
-				  const char *value ) {
+static int tftp_process_blksize ( struct tftp_request *tftp, char *value ) {
 	char *end;
 
 	tftp->blksize = strtoul ( value, &end, 10 );
  * @v value		Option value
  * @ret rc		Return status code
  */
-static int tftp_process_tsize ( struct tftp_request *tftp,
-				const char *value ) {
+static int tftp_process_tsize ( struct tftp_request *tftp, char *value ) {
 	char *end;
 
 	tftp->tsize = strtoul ( value, &end, 10 );
  * @v value		Option value
  * @ret rc		Return status code
  */
-static int tftp_process_multicast ( struct tftp_request *tftp,
-				    const char *value ) {
+static int tftp_process_multicast ( struct tftp_request *tftp, char *value ) {
 	union {
 		struct sockaddr sa;
 		struct sockaddr_in sin;
 	} socket;
-	char buf[ strlen ( value ) + 1 ];
 	char *addr;
 	char *port;
 	char *port_end;
 	int rc;
 
 	/* Split value into "addr,port,mc" fields */
-	memcpy ( buf, value, sizeof ( buf ) );
-	addr = buf;
+	addr = value;
 	port = strchr ( addr, ',' );
 	if ( ! port ) {
 		DBGC ( tftp, "TFTP %p multicast missing port,mc\n", tftp );
 	 * @v value	Option value
 	 * @ret rc	Return status code
 	 */
-	int ( * process ) ( struct tftp_request *tftp, const char *value );
+	int ( * process ) ( struct tftp_request *tftp, char *value );
 };
 
 /** Recognised TFTP options */
  * @ret rc		Return status code
  */
 static int tftp_process_option ( struct tftp_request *tftp,
-				 const char *name, const char *value ) {
+				 const char *name, char *value ) {
 	struct tftp_option *option;
 
 	for ( option = tftp_options ; option->name ; option++ ) {