Truncate instead of deleting log files

For package installs we need to be able to fix up permissions in
/var/log at install time, and if we delete all of the files then
those permission fixes get lost.  To deal with that, we can just
empty the files instead of deleting them.

Change-Id: I70abb354e4dc726ec7a176694c7c6e9455d89d08

elements/base/cleanup.d/99-tidy-logs

@@ -20,7 +20,7 @@ set -eux
 set -o pipefail
 
 # Truncate /var/log files in preparation for first boot
-sudo find $TARGET_ROOT/var/log -type f -delete
+sudo find $TARGET_ROOT/var/log -type f -exec cp /dev/null '{}' \;
 
 # also /root logs
 sudo find $TARGET_ROOT/root -name \*.log -type f -delete