Fix config files getting overwritten in opkg install

Commit 39ff0538

 accidentally removed the conffiles definitions
from luci-base, statistics, splash and diag-devinfo apps.

That has caused existing config files to get overwritten if the
package has been reinstalled via opkg. Additionally, the splash
text files have not been saved in sysupgrade or backup as they
are located outside /etc/config

This commit adds back the conffiles definitions to the respective
packages.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>