make post-synthesis results compatible with Vivado GUI

[sbourdeauducq]

Vivado GUI expects the output files, and maybe other things, to be placed in build/top.runs/impl_1. It currently does not find them as they are in build/ and maybe for other reasons, and wants to run synthesis and P&R again when you want to open the implemented design to make small changes to it (e.g. change a PLL phase).

[jordens]

If that works and teaches Vivado to recognize already existing outputs, then we can remove the DCP checkpoints.