[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/tests/gdbstub_test.gdb

@@ -3,16 +3,16 @@
 # Run:
 #   make bin/gpxe.hd.tmp
 #   make
-#   tests/gdbstub_test.gdb
+#   gdb
+#   (gdb) target remote :TCPPORT
+#   OR
+#   (gdb) target remote udp:IP:UDPPORT
+#   (gdb) source tests/gdbstub_test.gdb
 
 define gpxe_load_symbols
 	file bin/gpxe.hd.tmp
 end
 
-define gpxe_connect
-	target remote localhost:4444
-end
-
 define gpxe_assert
 	if $arg0 != $arg1
 		echo FAIL $arg2\n
@@ -78,7 +78,6 @@ define gpxe_test_step
 end
 
 gpxe_load_symbols
-gpxe_connect
 gpxe_start_tests
 gpxe_test_regs_read
 gpxe_test_regs_write