sys: Remove $FreeBSD$: one-line sh pattern

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

dtb: Be much less verbose in the building of dtb

Make the dtb/dtbo files with less noise. Remove echo statements, but add
back the call for the non-meta builds to replace the removed noise.

Suggetions by: jrtc27
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37073

Switch to the new device-tree vendor tree

The old vendor tree was never fully merged and doing partial merge isn't
supported with git subtree merge so a new one was created.
Switch the build to use the new DTS from sys/contrib/device-tree
This also bump the DTS used to be in sync with Linux 5.9
While here change the way to get the linux version, simply hardcode
the value in sys/dts/freebsd-compatible.dts and use awk to get that
to put it in the CFLAGS.
As a bonus we now have the bindings docs available
in sys/contrib/device-tree/Bindings/ so no need to link to the Linux repo
or to the vendor tree.

make_dtb.sh: Use $CPP instead of assuming that cpp is in $PATH

This fixes building in CheriBSD with a strict tmp path since we don't
bootstrap a cpp but pass the full path to clang-cpp instead.

While touching this file also fix all shellcheck warnings in make_dtb.sh.

Reviewed By: manu
Differential Revision: https://reviews.freebsd.org/D18376

Use ${ECHO} in dtb/dtbo build, pass in from dtb.mk for -s

Reported by: sbruno
MFC after: 3 days

Add dtb overlays support

DTB Overlays are useful to change/add nodes to a dtb without the need to
modify it.
Add support for building dtbo during buildkernel.
The goal of DTBO present in the FreeBSD source tree is to fill a gap in
time when we submit changes upstream (Linux). Instead of waiting 2 to 4 months
we can add a DTBO in tree in the meantime.
This is not for adding DTBO for capes/hat/addon boards, those will be
better to put in a ports.
This is also not for enabling a i2c/spi/pwm controller on certain pins,
each user have a different use case for those (which pins to use etc ...)
and we cannot have all possible configuration.

Add a dtbo for sun8i-h3-sid which add the SID node missing in upstream dts.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D14782