base-files: add option to make /var persistent

base-files: add option to make /var persistent

In OpenWrt, /var is symlinked to /tmp by default. This is done to reduce
the amount of writes to the flash chip, which often have not the
greatest durability. As a result, things like DHCP or UPnP lease files,
are not persistent across reboots.

Since OpenWrt can run on devices with more durable storage, it makes
sense to have an option for a persistent /var. Add an option to make
/var persistent. When enabled, /var will no longer be symlinked to /tmp,
but /var/run will be symlink to /tmp/run, as it should contains only
files that should not be kept during reboot. The option is off by
default, to maintain the current behaviour.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>