This bug was fixed in the package partman-btrfs - 18ubuntu1.1
--------------- partman-btrfs (18ubuntu1.1) xenial; urgency=medium
* On s390x, zipl bootloader cannot handle /boot on btrfs. LP: #1609140
-- Dimitri John Ledkov <email address hidden> Wed, 14 Dec 2016 12:19:38 +0000
This bug was fixed in the package partman-btrfs - 18ubuntu1.1
---------------
partman-btrfs (18ubuntu1.1) xenial; urgency=medium
* On s390x, zipl bootloader cannot handle /boot on btrfs. LP: #1609140
-- Dimitri John Ledkov <email address hidden> Wed, 14 Dec 2016 12:19:38 +0000