Feat/improve linker scripts

[FoniksFox]

Closes #625

[WIP]

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 yet (WIP):

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

To discuss:

• Should BoardInit have a weak definition or just be extern, so that the user has to implement it?
• Should System.c be moved to ST-LIB or not?
• Should rodata in ram be protected by the MPU? (Not implemente because of added complexity, and because that would need 6 extra regions and we only have 5 left, so we would need to invent something there...)