Bug#955469: initramfs-tools-core: Enable zstandard support

[Sedat Dilek <sedat.dilek@gmail.com>]

Hi,

thanks Norbert for taking care.

I had a similar approach for Debian's userland tools when testing
version 3 of "Add support for ZSTD-compressed kernel and initramfs"
against Linux v5.6.

Norbert's patch seems to include what I have in [1] (includes my
README_zstd-v3.txt).

I am running both ZSTD support on the kernel and userland side using a
ZSTD compressed initramfs image.

As a side-note: zstd-v4 is out for testing see [2] and [3].

Thanks.

Regards,
- Sedat -

[1] LINK: https://lore.kernel.org/lkml/CA+icZUXCn2an9aNDrm+-eneSAOyGibz0W1xYhwkA5k3B3U-5vQ@mail.gmail.com/
[2] Message ID: <20200401053913.216783-1-nickrterrell@gmail.com>
[3] https://github.com/terrelln/linux/commits/zstd-v4