Fixed compilation for ESP32 and ESP8266 with -Werror=unused-variable

[matthias-bs]

I am using OneWire in a larger project of mine (https://github.com/matthias-bs/BresserWeatherSensorTTN). When I added Travis-CI to improve the quality of my own source code, I came across the following problem:

Last command:  $  /home/travis/arduino-cli --format json compile --fqbn esp32:esp32:featheresp32:FlashFreq=80 --warnings all --dry-run /home/travis/Arduino/libraries/OneWire/examples/DS18x20_Temperature/DS18x20_Temperature.ino

/home/travis/Arduino/libraries/OneWire/OneWire.cpp: In member function 'uint8_t OneWire::reset()':

/home/travis/Arduino/libraries/OneWire/OneWire.cpp:169:24: warning: unused variable 'reg' [-Wunused-variable]

  volatile IO_REG_TYPE *reg IO_REG_BASE_ATTR = baseReg;

                        ^~~

/home/travis/Arduino/libraries/OneWire/OneWire.cpp: In member function 'void OneWire::write_bit(uint8_t)':

/home/travis/Arduino/libraries/OneWire/OneWire.cpp:204:24: error: unused variable 'reg' [-Werror=unused-variable]

  volatile IO_REG_TYPE *reg IO_REG_BASE_ATTR = baseReg;

                        ^~~

/home/travis/Arduino/libraries/OneWire/OneWire.cpp: In member function 'uint8_t OneWire::read_bit()':

/home/travis/Arduino/libraries/OneWire/OneWire.cpp:232:24: error: unused variable 'reg' [-Werror=unused-variable]

  volatile IO_REG_TYPE *reg IO_REG_BASE_ATTR = baseReg;

                        ^~~

cc1plus: some warnings being treated as errors

It seams that the variable reg is used in macros on some architectures, but not on ESP32 and ESP8266. The default setting for arduino-ci and other CI implementations based on arduino-cli is to treat unused variables as errors, which breaks my CI test.
There does not seem to be an easy way to treat included libraries in a different way than your own code.

As a workaround, I added #pragma statements to temporarily switch the compiler to treat the unused variable as warning (just for the offending lines).

Another solution would be to declare the variables only for the architectures which need them.

[matthias-bs]

Logfile:
https://app.travis-ci.com/github/matthias-bs/OneWire/builds/255255423
(I hope this is visible to anyone...)

[matthias-bs]

Issue #74 seems to propose another solution.

[uzi18]

@matthias-bs just switch to OneWireNg

[matthias-bs]

@uzi18: Thanks for the proposal! Done!

[mathertel]

Is resolved in the master branch by using __attribute__((unused)).

[brentru]

@PaulStoffregen #120 seems to do the same thing as this PR proposes. Would it be possible for you or another maintainer to cut a release from master and close out this PR?