templated linker scripts

[kilograham]

I thought there was an issue for this already, but there isn't;

We should generate actual linker scripts from templates at build time, allowing for substitution of board parameters (e.g. PICO_FLASH_SIZE), but also either making it a) less brittle to make your own linker script (because you can easily compose the standardized pieces) in your own template b) easy to modify the existing linker scripts, by having the stock linker script templates be rich enough for customization by variables.

Note the overall goal here for a) & b) is to prevent people copying the existing flat scripts and customizing them (which is very brittle)