backup: Create tarball in one pass

backup: Create tarball in one pass

This patch is changing the behaviour of the backup script so that it
creates one tarball and compresses it in one go.

This will save storing the original tarball on disk before compressing
it which on my test system requires significant disk space.

This patch also solves a bug where the backup file included with the ISO
image could not be extracted because it was not gzip-compressed when it
was expected to be.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>