Add gdb stub library

libraries/GDBStub/README.md

@@ -0,0 +1,49 @@
+## Using GDB stub
+
+- Add `#include <GDBStub.h>` to the sketch
+- Upload the sketch
+- Redirect serial port to TCP port:
+```
+    tcp_serial_redirect.py -p /dev/tty.SLAB_USBtoUART -b 115200 --spy -P 9980 --rts=0 --dtr=0
+```
+Change port and baud rate as necessary. This command requires python and pyserial.
+- Observe serial output:
+```
+    nc localhost 9980
+```
+- When crash happens, `Trap %d: pc=%p va=%p` line will appear in serial output.
+- Close nc and start gdb:
+```
+    xtensa-lx106-elf-gdb /path/to/Sketch.cpp.elf -ex "target remote :9980"
+```
+- Use gdb to inspect program state at the point of an exception.
+
+## Tips and tricks
+
+- To halt the target when software WDT fires, add
+```
+    ((int*)0) = 0;
+```
+at the top of `__wrap_system_restart_local` in core_esp8266_postmortem.c.
+
+## License
+
+GDB Server stub by Marko Mikulicic was taken from Cesanta's smart.js
+
+https://github.com/cesanta/smart.js
+
+Copyright (c) 2013-2014 Cesanta Software Limited
+All rights reserved
+
+This software is dual-licensed: you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as
+published by the Free Software Foundation. For the terms of this
+license, see <http://www.gnu.org/licenses>.
+
+You are free to use this software under the terms of the GNU General
+Public License, 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.
+
+Alternatively, you can license this software under a commercial
+license, as set out in <https://www.cesanta.com/license>.