stm32l4x6: implement flash storage server

[jmaksymowicz]

Description

This PR adds a flash storage server to the STM32 multiserver. It allows data to be read from and written to flash storage through a block device, as well as mounting of filesystems. Currently only meterfs is supported, but LittleFS support is planned in the future.

The file server can be disabled through compile time flags. If the file server will be used to mount root filesystem, another compile time flag disables the use of dummyfs for that purpose (in that case dummyfs is only used for creating devfs).

Partition and file system setup is done through command line arguments in the user script.

Motivation and Context

With this PR it is possible to use a filesystem and persistently store files on STM32 targets.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: armv7m4-stm32l4x6-nucleo

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 284 tests  ±0   6 569 ✅ ±0   37m 54s ⏱️ + 1m 59s
  409 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 3ab7275. ± Comparison against base commit 44531d9.

♻️ This comment has been updated with latest results.

[lukileczo]

The recent msg API changes affect this PR, summary in phoenix-rtos/phoenix-rtos-project#1035