Additional software: support read-only persistence

Apt wants to create /var/cache/archive/pkgcache.bin near the root of an aufs mount (but outside it). Unfortunaly, the kernel doesn’l like that.

# touch /var/cache/apt/packages.bin
aufs au_lkup_neg:326:touch[<PID>]: I/O Error, apt should be negative on b0

Original created by @tails on 5815 (Redmine)

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