As a follow up toc461e20df3
("make: Remove version from build output path"), only support building a single version of a single board by default. The build directory can still be specified by passing `BUILD`. Fixes running the `clean` target when `BUILD` is a custom value or `BOARD` isn't set. Fixes:b03c960b4f
("make: Reduce build output") Signed-off-by: Tim Crawford <tcrawford@system76.com>