Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

release: Add support for building on non-FreeBSD

This requires two sets of changes. Firstly, for non-FreeBSD, we do not
know where tools are in PATH (and it is likely that some are not in
system directories and have been built as bootstrap tools during the
build), so we should leave PATH alone and trust the user. Secondly,
makefs needs a master.passwd for building images from a METALOG file, so
pass the directory in the image tree to makefs's -N option in order to
pick up a valid FreeBSD master.passwd; this is unnecessary on FreeBSD
(except in the edge case of building an image that refers to users or
groups not present in the host's database, which is unlikely but
technically possible) but harmless so can be done unconditionally.

Reviewed by: brooks, emaste, gjb
Differential Revision: https://reviews.freebsd.org/D34001

release: Support -DNO_ROOT image building

This requires a bunch of METALOG mangling to include the files we inject
into the tree. The mkisoimages.sh and make-memstick.sh scripts are now
called with the current directory inside the tree so that the relative
paths in the METALOG match up with the current directory. The scripts do
not require this when not using a METALOG, but for simplicity we always
do so. The Makefile mangles the real METALOG created from the install,
as those files are shared across all uses of the tree, but the shell
scripts create a temporary copy of the METALOG that they mangle as their
tree modifications are specific to that image. We also need to pass -D
to makefs to turn any duplicate METALOG entry errors into warnings, as
we have many (harmless) instances of those.

Whilst dvd1.iso should work, the !NOPKG code will need more work to
support this.

All media will also lack mergemaster and etcupdate trees, since more
work is needed to add -DNO_ROOT modes to them. Users of install media
built this way will have to manually bootstrap them.

Reviewed by: brooks, gjb
Differential Revision: https://reviews.freebsd.org/D33999

Copy arm64 make-memstick.sh and mkisoimages.sh to the riscv
directory to allow properly building *.iso and *.img files.

Sponsored by: Rubicon Communications, LLC (netgate.com)