image_types_phosphor.bbclass: mmc tar: Compress ext4 rootfs

Compress the ext4 rootfs so that it's faster to transfer to the
BMC and does not take much space. The code update process can
then uncompress it on the fly and write to flash.

Use zstd to compress which has a good compression rate and is also
faster to decompress than xz.

(From meta-phosphor rev: 5e1b6dc499736e219766cc24924dd486ff2a8ccb)

Change-Id: I07aadc413e5e883a6b97d1a374e4bbf215b07786
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>