Do not treat custom menu options without label as 'malformed'

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?
  Do not report the board menu without a label as an error. It happens when the boards.txt does not specify a top-level menu.xxx directive, for example:

#menu.FlashSize=Flash Size    <--- removing this line will trigger the error described in this PR
[....]
m5stack-core-esp32.menu.FlashSize.4M=4MB (32Mb)
m5stack-core-esp32.menu.FlashSize.4M.build.flash_size=4MB
m5stack-core-esp32.menu.FlashSize.16M=16MB (128Mb)
m5stack-core-esp32.menu.FlashSize.16M.build.flash_size=16MB

• What is the current behavior?
  If the menu directive is missing the CLI fails to load the whole platform with the message:

Error initializing instance: Error loading hardware platform: loading platform release xxxx@x.y.z: loading boards: skipping loading of boards xxxxxx: malformed custom board options

• What is the new behavior?
  The CLI will silently accept the board option without the label.

• Does this PR introduce a breaking change, and is titled accordingly?
  Not exactly a breaking change, but worth noting that the gRPC response to BoardDetails now may have a ConfigOption with a blank option_label field:

message ConfigOption {
  // ID of the configuration option. For identifying the option to machines.
  string option = 1;
  // Name of the configuration option for identifying the option to humans.
  string option_label = 2;
  // Possible values of the configuration option.
  repeated ConfigValue values = 3;
}

[codecov]

Codecov Report

Base: 36.68% // Head: 36.58% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (ec5bbd5) compared to base (7d1916d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1882      +/-   ##
==========================================
- Coverage   36.68%   36.58%   -0.11%     
==========================================
  Files         231      231              
  Lines       19680    19677       -3     
==========================================
- Hits         7219     7198      -21     
- Misses      11633    11648      +15     
- Partials      828      831       +3     

Flag	Coverage Δ
unit	36.58% <ø> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
arduino/cores/packagemanager/loader.go	72.30% <ø> (-2.01%)	⬇️
internal/integrationtest/arduino-cli.go	79.85% <0.00%> (-2.73%)	⬇️

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