debian: exclude contents of /tmp from debootstrap tarballs

Skip the contents of /tmp/ when creating tarballs with the results of
debootstrap, as that directory contains just ccache cache (and in
general temporary files).

Small (positive) side effect is that deboostrap tarballs gets slightly
smaller, something like 2-3 MB less.

Change-Id: Ib4adf3dc908f02deed408bf9188f032767eeb2bb

elements/debian/root.d/08-debootstrap

@@ -98,7 +98,7 @@ iface eth0 inet dhcp
 EOF
 
     echo Caching debootstrap result in $DEBOOTSTRAP_TARBALL
-    sudo tar -C $TARGET_ROOT -zcf $DEBOOTSTRAP_TARBALL .
+    sudo tar -C $TARGET_ROOT -zcf $DEBOOTSTRAP_TARBALL --exclude='./tmp/*' .
 fi
 
 sudo rm -f ${TARGET_ROOT}/.extra_settings