GDBStub example using PlatformIO

See: esp8266/Arduino#4386

Instructions

• Change the port in the last line of gdbcmds from /dev/cu.SLAB_USBtoUART to your port.
  • On mac/linux, run ls /dev/cu.* if you're not sure which port.
• Upload firmware to you board: platformio run -t upload
• Connect gdb: ./gdb.sh