[GDB] Remote debugging over UDP

This commit implements GDB over UDP.  Using UDP is more complex than
serial and has required some restructuring.

The GDB stub is now built using one or both of GDBSERIAL and GDBUDP
config.h options.

To enter the debugger, execute the gPXE shell command:
gdbstub <transport> [<options>...]

Where <transport> is "serial" or "udp".  For "udp", the name of a
configured network device is required:
gdbstub udp net0

The GDB stub listens on UDP port 43770 by default.

src/core/gdbserial.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2008 Stefan Hajnoczi <stefanha@gmail.com>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <assert.h>
+#include <gpxe/serial.h>
+#include <gpxe/gdbstub.h>
+
+struct gdb_transport serial_gdb_transport __gdb_transport;
+
+static size_t gdbserial_recv ( char *buf, size_t len ) {
+	assert ( len > 0 );
+	buf [ 0 ] = serial_getc();
+	return 1;
+}
+
+static void gdbserial_send ( const char *buf, size_t len ) {
+	while ( len-- > 0 ) {
+		serial_putc ( *buf++ );
+	}
+}
+
+struct gdb_transport serial_gdb_transport __gdb_transport = {
+	.name = "serial",
+	.recv = gdbserial_recv,
+	.send = gdbserial_send,
+};