Improvement of backup iso script

Improvement of backup iso script

The backup iso script did not check the arch of the host. On x86_64 host
the wrong iso was downloaded.

Furthermore, there were some if clauses which could cause trouble which
I also tried to improve.
(For example: -e is valid if we have a directory or a file, but we want
to check for a file only )

Fixes: 11258
Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>