make reproduceable tar files using GNU tar specifics

https://wiki.debian.org/ReproducibleBuilds
make pkgdetails and busybox build repeatable on my machine

Using `faketime` means the timestamps are forced to being the same, and
that is enough to make the builds bit-for-bit repeatable on my machine.
Hopefully it is enough for repeatable builds on other machine as well.
use faketime on release builds for deterministic builds

Running a program with faketime causes that program to recent a fixed time
based on the timestamp provided to faketime.  This ensures that the
timestamps in the files are always the same.

This gets the process to the point where the APK signature is transferrable
no matter who builds it.  This process does not produce an APK with the
same hash.