Move the BOOM configs to the boom submodule, default to BOOMConfig

With this commit you'll no longer be able to run upstream Rocket Chip
configs by just running

  make CONFIG=SomeRocketChipConfig

instead you'll need to run

  make CONFIG=SomeRocketChipConfig CFG_PROJECT=rocketchip

the advantage is that the boom branch will no longer require any diff
against rocket-chip aside from some default Makefile variables and a
submodule pointer.

Be sure to look at

  https://github.com/ucb-bar/riscv-boom/pull/15

before merging this.