Improve GDBStub documentation

[boguslawb]

Can you prepare a tutorial (video of actual working task would be ideal) how exactly to use GDBStub to catch and backtrace exception ? I followed your description and found the same issues as here http://blog.squix.org/2016/04/esp8266-offline-debugging-with.html
gdb cannot attach to remote process, if I try directly attach to redirected I got invalid packet error in gdb, if I try to add line ((int*)0) = 0 then I cannot run device - it stuck on reset

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[tijnkooijmans]

Same issue here, this is my output from xtensa-lx106-elf-gdb:

Reading symbols from /Users/tijn/Develop/minibrew-firmware/ESP8266/MiniBrewWifi/Builds/embeddedcomputing.elf...done.
Remote debugging using :9980
Ignoring packet error, continuing...
warning: unrecognized item "timeout" in "qSupported" response

[earlephilhower]

GDB support seems to require both some toolchain fixes coupled with some libraries/GDBstub changes as well.

[earlephilhower]

This is done in PR #5559 where we have a new GDBStub and a writeup in the documentation directory. We'll track progress in that pull.