MFV r318941: 7446 zpool create should support efi system partition

MFV r318941: 7446 zpool create should support efi system partition

illumos/illumos-gate@7855d95b30fd903e3918bad5a29b777e765db821
https://github.com/illumos/illumos-gate/commit/7855d95b30fd903e3918bad5a29b777e765db821

https://www.illumos.org/issues/7446
  Since we support whole-disk configuration for boot pool, we also will need
  whole disk support with UEFI boot and for this, zpool create should create efi-
  system partition.
  I have borrowed the idea from oracle solaris, and introducing zpool create -
  B switch to provide an way to specify that boot partition should be created.
  However, there is still an question, how big should the system partition be.
  For time being, I have set default size 256MB (thats minimum size for FAT32
  with 4k blocks). To support custom size, the set on creation "bootsize"
  property is created and so the custom size can be set as: zpool create B -
  o bootsize=34MB rpool c0t0d0
  After pool is created, the "bootsize" property is read only. When -B switch is
  not used, the bootsize defaults to 0 and is shown in zpool get output with
  value ''. Older zfs/zpool implementations are ignoring this property.
  https://www.illumos.org/rb/r/219/

Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Approved by: Dan McDonald <danmcd@kebe.com>
Author: Toomas Soome <tsoome@me.com>

This commit makes no sense for FreeBSD, that is why I blocked the option,
but it should be good to stay closer to upstream.