Add option to specify name of build directories (both litex and software)

On a multi-core machine, design space exploration (DSE) may want to kick of multiple parallel jobs. We don't want to require multiple clones of the repo and tool installations; we'd like to spawn multiple builds in the same repo with with same tool installation. To do so would require the ability to set the name of the build directories (both the LiteX build under `soc/build/`, and the software build under `proj/projname/build/`.

FYI @ShvetankPrakash