linux: qstrip the custom DTS path

This fixes a problem reported by wacha@mit.bme.hu, see
http://lists.busybox.net/pipermail/buildroot/2013-June/073312.html

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Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>