patch:  Enable multiple RTAI versions on a single system

/usr/bin/rtai-config becomes a stub script that helps select another
kernel-specific /usr/bin/rtai-config-<kver>.

https://github.com/ShabbyX/RTAI/issues/16
rules:  ignore failed 'make distclean'

'make distclean' doesn't ever work, so try it but ignore failures