LIVE-545 Add feature flag for enabling USB firmware update on LLM

[grsoares21]

Context (issues, jira)

LIVE-545

Description / Usage

This branch creates the definition of a new feature flag to be used for the mobile firmware update feature.
It also puts string as a possible type for feature flags in order to allow us to develop even without upgrading live-common.

Expectations

• Test coverage: The changes of this PR are covered by test. Unit test were added with mocks when depending on a backend/device.
• No impact: The changes of this PR have ZERO impact on the userland. Meaning, we can use these changes without modifying LLD/LLM at all. It will be a "noop" and the maintainers will be able to bump it without changing anything.

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[thomasrogerlux]

I have mixed feelings about the string addition but okay

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/22.1.x@216c3e5). Click here to learn what that means.
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❗ Current head 644f2b1 differs from pull request most recent head b2f469b. Consider uploading reports for the commit b2f469b to get more accurate results

@@                Coverage Diff                @@
##             release/22.1.x    #1843   +/-   ##
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  Hits                      ?    12537           
  Misses                    ?     8548           
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Name	Status	Preview	Updated
ledger-live-common	❌ Failed (Inspect)		May 5, 2022 at 1:32PM (UTC)