Update condition on downloading binary file(s)

Since upstream v7.45-2, it requires binary files for both architectures
again. This patch updates the condition on detecting whether we need
both files or just one for the version we are building against.
Run upstream's preparing script if it exists

Upstream v7.45-2 introduced a preparing script to download and inflate
archives. If the version we are building against contains that script,
then run it to inflate archives in order to build packages.
Merge remote-tracking branch 'ladios/7u51-fix' into trusted-key