Undefine F and shadow __FlashStringHelper

[facchinm]

This PR "competes" with #82 and #85 to understand which is more convenient based on CI results.
Before running CI a0f6beb was applied so a baseline with no compilation error exists.

arduino/arduino-examples@b94e757#r44269343 need to be addressed for this PR to compile successfully

[facchinm]

Fails on CI for overloads like

2020-12-09T08:58:08.2397342Z In file included from /github/home/Arduino/libraries/Arduino_DebugUtils/examples/Arduino_Debug_Advance/Arduino_Debug_Advance.ino:11:0:
2020-12-09T08:58:08.2399215Z /github/home/Arduino/libraries/Arduino_DebugUtils/src/Arduino_DebugUtils.h:60:10: error: 'void Arduino_DebugUtils::print(int, const char*, ...)' cannot be overloaded
2020-12-09T08:58:08.2400151Z      void print(int const debug_level, const __FlashStringHelper * fmt, ...);
2020-12-09T08:58:08.2400608Z           ^~~~~
2020-12-09T08:58:08.2402030Z /github/home/Arduino/libraries/Arduino_DebugUtils/src/Arduino_DebugUtils.h:59:10: error: with 'void Arduino_DebugUtils::print(int, const char*, ...)'
2020-12-09T08:58:08.2402742Z      void print(int const debug_level, const char * fmt, ...);
2020-12-09T08:58:08.2403093Z           ^~~~~
2020-12-09T08:58:08.2403615Z Using library Arduino_DebugUtils at version 1.1.0 in folder: /github/home/Arduino/libraries/Arduino_DebugUtils 
2020-12-09T08:58:08.2404644Z Using library SoftwareSerial at version 1.0 in folder: /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/libraries/SoftwareSerial 
2020-12-09T08:58:08.2405337Z Error during build: exit status 1
2020-12-09T08:58:10.8990229Z 

need to find a better solution...