Improved {build.*.platform.path} vars enumeration.

[cmaglie]

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What kind of change does this PR introduce?

This is a tentative fix for #2616. The original issue provided an experimental platform that extends esp32 to experiment with.
This PR adds the following template variables that will be available to compose recipes:

• {build.core.platform.path} contains the path to the reference (build) platform
• {build.board.platform.path} contains the path to the board platform

Also, adds the input variable build.use_core_platform_for_runtime_platform_path to be set on boards.txt or platform.txt:

• if set to true then the value of {runtime.platform.path} will be the path to the reference platform.
• in all other cases (unset or !=true) the value of {runtime.platform.path} will be the path to the board platform (as it always has been)

What is the current behavior?

The compile fails with an "extended" platforms

What is the new behavior?

Unfortunately, the compile still fails. Hopefully, this PR could help to resolve the remaining issues.

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

Other information

/cc @egnor

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.22%. Comparing base (850f22a) to head (7af9663).

Files	Patch %	Lines
...al/arduino/cores/packagemanager/package_manager.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   70.23%   70.22%   -0.01%     
==========================================
  Files         222      222              
  Lines       21347    21352       +5     
==========================================
+ Hits        14993    14995       +2     
- Misses       5175     5177       +2     
- Partials     1179     1180       +1     

Flag	Coverage Δ
unit	70.22% <40.00%> (-0.01%)	⬇️

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[egnor]

(kibitz: build.use_core_platform_for_runtime_platform_path might not be quite right, since it doesn't just affect the build but in fact just about any variable, including upload and so on?)