Integrate HAVE_MCU_SERIAL_STORAGE flag code

[nroggeman-ledger]

Description

The goal of this PR is to Integrate and remove HAVE_MCU_SERIAL_STORAGE (always defined) flag.
This is a PR of code cleaning.
It is not planned to support products or flavors without this feature, so the flag is useless (it was defined only for Nanos but it was a mistake, because available on all products)

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• [*] Other (for changes that might not fit in any category)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.49%. Comparing base (b1727b3) to head (299ed47).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #836   +/-   ##
=======================================
  Coverage   35.49%   35.49%           
=======================================
  Files          13       13           
  Lines        1606     1606           
=======================================
  Hits          570      570           
  Misses       1036     1036           

Flag	Coverage Δ
unittests	35.49% <ø> (ø)

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