MAKEALL: Fix kill_children

When building in parallel, make sure that we look up the children
based on the the actual process group id instead of just assuming
that the MAKEALL pid is the process group id.

Also ensure that logs from incomplete builds are deleted in the
process.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>