Dynamic linker generation

[perigoso]

I'm working on the non volatile storage right now, which involves reserving sections of memory,
we really only need to reduce the rom size by how much space we want to reserve and provide a symbol or two, but having a dynamic linker would still be nice, there's also things like possible different offsets for the code for different bootloader sizes etc that would benefit from this.
We can hand edit linkers for each target so we aren't really limited right now anyway.