netboot creation: no longer compress the tarball + only generate sha256 checksum...

netboot creation: no longer compress the tarball + only generate sha256 checksum file

Compressing the tarball takes several seconds, while there's usually
no change in file size, as pretty much all its files are already
compressed. By no longer compressing the tarball we reduce build time.

Also no longer generated sha1 and sha512 checksum files, we agreed
to only rely on sha256 checksum files nowadays.

Related to https://github.com/grml/grml/issues/120