board/boundarydevices: update 6x_bootscript to use zImage

As suggested by Thomas:
http://lists.busybox.net/pipermail/buildroot/2015-September/139884.html



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>