Add pointercal installation
Xeon only builds for one device. I can either drop the pointercal (already generated on the device with ts_calibrate) in <buildroot>/skeleton/etc/pointercal or put it under <buildroot>/skeleton/usr/share/pibox/calibrate and install it with optimizeTouchscreen() on firstboot. The latter is what the PiBox systems do but they support multiple hardware. Xeon is just the PinePhone Pro at the moment.