Print FQBN in verbose compile output

[cmaglie]

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What kind of change does this PR introduce?

Prints the FQBN at the beginning of the verbose compile output.
This is useful when compiling from the Arduino IDE to easily get an FQBN suitable for the arduino-cli.

What is the current behavior?

What is the new behavior?

$ arduino-cli compile -b arduino:avr:mega:cpu=atmega1280 -v
FQBN: arduino:avr:mega:cpu=atmega1280
Utilizzo della scheda 'mega' dalla piattaforma nella cartella: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6
Utilizzo del core 'arduino' dalla piattaforma nella cartella: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6

Rilevamento delle librerie utilizzate in corso...
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega1280 -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_MEGA -DARDUINO_ARCH_AVR -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/mega /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp -o /dev/null
Alternative per Wire.h: [Wire@1.0]
ResolveLibrary(Wire.h)
  -> candidati: [Wire@1.0]
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega1280 -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_MEGA -DARDUINO_ARCH_AVR -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/mega -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire/src /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp -o /dev/null
...

Does this PR introduce a breaking change, and is titled accordingly?

Other information

Fix #2117

[codecov]

Codecov Report

Patch coverage: 78.12% and project coverage change: +0.03 🎉

Comparison is base (5a67f3b) 62.62% compared to head (532f3fd) 62.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2134      +/-   ##
==========================================
+ Coverage   62.62%   62.65%   +0.03%     
==========================================
  Files         231      231              
  Lines       19610    19648      +38     
==========================================
+ Hits        12281    12311      +30     
- Misses       6229     6234       +5     
- Partials     1100     1103       +3     

Flag	Coverage Δ
unit	62.65% <78.12%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
legacy/builder/target_board_resolver.go	75.00% <55.55%> (-5.65%)	⬇️
arduino/cores/packagemanager/package_manager.go	75.24% <81.25%> (+1.58%)	⬆️
arduino/cores/fqbn.go	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[ubidefeo]

FQBN is correctly printed to IDE's output.
LGTM