preserve calibration data during rauc updates

Only the bbb-based calibration-box currently uses these. I think we'll
move stuff to EEPROMs, eventually, but before that happens, let's make
sure the data are not lost upon update.

I forgot to add this one because the first bring-up used just that SD
card, so there were no "updates" to speak of.

Change-Id: I669547cd88d25779e59486c702782a62afa79756
diff --git a/package/czechlight-rauc/rauc-hook.sh b/package/czechlight-rauc/rauc-hook.sh
index b6ace5c..64a536a 100755
--- a/package/czechlight-rauc/rauc-hook.sh
+++ b/package/czechlight-rauc/rauc-hook.sh
@@ -6,6 +6,7 @@
       cfg)
         # whitelist so that we don't copy cruft or lost+found
         for ITEM in \
+            calibration \
             etc \
             journald-remote \
             random-seed \