feat: enable persistent var (EXPERIMENTAL)

[gsanchietti]

EXPERIMENTAL: test how the image behaves

Upstream commit: openwrt/openwrt@57807f5

[gsanchietti]

The persistent var could be useful to retain the following files:

• /var/log/lastlog is always empty
• /var/log/messages is already stored also in /mnt/data
• /var/log/wtmp could be useful, but we do not have the last command to parse it, also similar info is already present inside the messages

But has many drawbacks:

• upstream implementation does not support mounting a different partition under /var
• it stores also many files in /vat/etc that are recreated by applications upon starts, such files should be temporary
• the messages file can get bigger and fill all the root
• victoria metrics are stored inside /var/lib/victoria-metrics-data and can get bigger and fill the root
• despite of the linked commit, having a persistent var does not preserve dhcp leases across reboots because they are saved by default on a temp directory (dhcp.ns_dnsmasq.leasefile='/tmp/dhcp.leases')

In the end, I think that persistent may cause many problems on long running machines.
My opinion is to ditch the feature and eventually move the DHCP lease to storage, when present.