Feat/improve linker scripts

[FoniksFox]

Closes #625
Closes #653
Closes #658

A refactor of the Linker Script, Startup Code and System.c

Check the corresponding pr in the template-project to see the changes needed to make it work.

It implementes:

• Initialized data sections for all memory domains with a copy table, so that ResetHandler can generally copy all data defined in the table.
• A Linker Script with configurable ITCM memory via macros (C preprocessor) and the possibility to add all code to ITCM.
• ISR vector always copied to ITCM.
• Implement DTCM constants.
• weak BoardInit function that gets called after SystemInit, but before global constructors

Not implemented left as a weak definition so that users can implement their own version):

• Configuration checker (a function to check whether the provided ITCM size matches the defined size, and whatever other configuration that's deemed as necessary)

[github-actions]

ST-LIB Release Plan

• Current version: 6.1.3
• Pending changesets: 1
• Highest requested bump: major
• Next version if merged now: 7.0.0

Pending changes

• major Refactor linker script, startup code, and memory model with unified copy/zero tables, configurable ITCM, and weak BoardInit (.changesets/refactor-linker-script.md)