Add ESP8266 Support

[brentru]

Adding support for ESP8266 Deep Sleep + Software WDT

• As noted in utility/WatchdogESP8266.cpp, configuring the software WDT timeout maxPeriodMS value is NOT
  IMPLEMENTED in the ESP8266 BSP [1]. Further, an investigation into the default software WDT time yielded a fixed timeout period of 3.2 seconds [2].
  [1] https://github.com/esp8266/Arduino/blob/master/cores/esp8266/Esp.h#L91
  [2]https://sigmdel.ca/michel/program/esp8266/arduino/watchdogs_en.html#ESP8266_WDT_TIMEOUT
  • Therefore, calls to enable() for this board will use the default, fixed, timeout period regardless of the period passed in.
  • This caveat has been noted in-code and on the README.md
• examples/BasicUsage/BasicUsage.ino has been modified to implement delayMicroseconds on ESP8266 arch. as calling delay() feeds the WDT, loop() will always be reached as the WDT is fed.