soc/builder: add initialize_memory()

[AndrewD]

Allows the target soc to override memory initialization:
rom and/or ram.

Includes documentation for example usage to compile soc target specific software and load it to main_ram. This mechanism
can also be used to load a custom bios into ROM instead of the standard Litex bios.

[enjoy-digital]

Sorry for the delay in the review @AndrewD. I'll review it now and will probably modify things a bit. I also have a preference to avoid that much documentation/example directly in the code and prefer having it in a wiki. I'll do the change and will provide an update here.

[enjoy-digital]

@AndrewD: I've reworked things a bit on top of your commit with:

• f181eab
• 9f49b13
• 175aab7
• 06083c7
• 31508dd
• 38a8a17
  Can you check it still allow you to do what you wanted?

For an example of the usage, we could add it to the wiki or to a specific design requiring it.

[AndrewD]

I was thinking of documenting some usage in the wiki, for example this can improve the demo firmware integration.
I used this for much cleaner integration of unit testing firmware, but it can also be used to load, for example, a customised bios for a target.