Add dtb overlays support

[manu]

Summary

DTB Overlays are useful to change/add nodes to a dtb without the need to modify it.
Add support for building dtbo during buildkernel.
Add a dtbo for sun8i-h3-sid which add the SID node missing in upstream dts.
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.

[kevans]

This seems to hit all of the things I care about, at least: installed to /boot/dtb/overlays with /compatible specified on the overlay to make sure we're not producing .dtbo's that can easily be applied to the wrong board.