Make boot ROM contents configurable through top-level parameter

[olofk]

This makes it easier for external tools to change boot ROM

[sam-falvo]

Thanks! Parameterization is something I don't have much experience with, but will need to research going further.

I hear that Vivado does not support readmemh() though when instantiating block RAMs. This is a horrible design decision on Xilinx's part. What are they thinking?! I'll have to think of something else for a computer that is actually synthesizable.

[olofk]

Are you sure about that? I'm pretty convinced that works

[sam-falvo]

Jan Gray had problems with his GRVI project, according to what he expressed during the RISC-V workshop. Dunno; I don't use Vivado.