Adjust HDMI / custom dtb docs

Since our switch to flash-kernel https://developer.puri.sm/Librem5/Development_Environment/Boards/imx8.html#hdmi does not work anymore. See

https://lists.community.puri.sm/pipermail/librem-5-dev/2019-May/000177.html

(and all other places that mess with dtbs). DTBs should go into /etc/flash-kernel/dtbs/ and we should a note that this might break since we don't consider or DT stable (e.g. it will break when moving to 5.x).

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