Improve MAKEALL parallel builds

The patch that added parallel builds broke MAKEALL -l, so this
fixes that. At the same time, it improves the termination so
that it shuts down the build threads if you cancel the build.
Lastly, it removes a bunch of debug code.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Tested-by: Wolfgang Denk <wd@denx.de>