rules.featureset-xenomai:  fix checking of ipipe patch existence
WIP:  Add RTAI featureset

This is a WIP:

- rtai-config script is broken
- first try at Machinekit build failed:

  CC [M]  /tmp/linuxcnc-2.6.0/src/objects/rtai-kernel/3.5.0-1mk-rtai-amd64/emc/kinematics/5axiskins.o
make[2]: Leaving directory `/usr/src/linux-headers-3.5.0-1mk-rtai-amd64'
/tmp/linuxcnc-2.6.0/src/objects/rtai-kernel/3.5.0-1mk-rtai-amd64/emc/kinematics/5axiskins.c: In function ‘s2r’:
/tmp/linuxcnc-2.6.0/src/objects/rtai-kernel/3.5.0-1mk-rtai-amd64/emc/kinematics/5axiskins.c:32:18: error: SSE register return with SSE disabled
make[5]: *** [/tmp/linuxcnc-2.6.0/src/objects/rtai-kernel/3.5.0-1mk-rtai-amd64/emc/kinematics/5axiskins.o] Error 1