[mk] Add a 'clean-build' toplevel target, for 'checkout' packages
This runs a `make clean` inside the package source. This is probably only useful for packages that are 'checkout', where `make clean` can't run. While here, let `make cleaner` invoke this target for a checkout package, instead of doing nothing. The clean-build target can be customized through CLEAN_TARGET (or CLEAN_TARGET.<build dir>) and CLEAN_MAKE_CMD. CLEAN_MAKE_FLAGS is also available, defaulting to BUILD_MAKE_FLAGS.