heir: Document SYSROOT conventions

Define a place for sysroot trees to live. This assumes they come from
the base in some way, though there's not yet a build/install/etc sysroot
target. Include the FreeBSD version so multiple verrsions can be
installed on one system (it also includes the whole uname version, so
one could, in theory, install variants like CheriBSD or whatever on the
same system as FreeBSD). Use MACHINE.MACHINE_ARCH to be consistent with
the release practices, /usr/obj and other naming conventions.

Sponsored by: Netflix
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D33754