busybox

- work around missing features and bugs in make-3.79.1:
- no order-only prerequisites; incomplete workaround (see comment in diff)
- no internal variable MAKEFILE_LIST; workaround
- define bar\nfoo:=<long-list>  doesn't work; workaround
also:
- reinstate clean, distclean as noconfig_target.
- unconditionally clean libbusybox.so*