MFC r293188:
Prevent memstick installation medium from attempting to mount
the root filesystem read-write. This causes problems booting
the memstick installation medium from write-protected USB flash
drives.

PR: 187161, 205886
Sponsored by: The FreeBSD Foundation

MFC r265171 by nwhitehorn: Use mkimg instead of md(4) and gpart [for powerpc].

Approved by: re

MFC r265171 by nwhitehorn: Use mkimg instead of md(4) and gpart [for powerpc].

Approved by: re