MFC: r285594
New partition flag for gpart, writes the 0xee partition in the pmbr in the second slot, rather than the first.
Works around Lenovo legacy GPT boot issue

PR: 184910
Approved by: re (gjb), marcel
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D3140