Adds a BUILD_BASE_DIR makefile variable that allows you to specify the
base directory for FPGA build outputs. This allows you to put the
output from each run of make into a unique directory, or to run
multiple instances of make in parallel with each build's output being
put in a different directory. For example:
make X310_XG BUILD_BASE_DIR=/path/to/builds
Original-commit: bc6db8409ba756bbad0d0c158ffb00c614b58fd9
Wade Fife