librem5-devkit: Ship a devicetree with the upstream name

This will allow us to move flash-kernel to the upsteam name as well and
people upgrading to 5.x will get the right device tree with a recent
flash-kernel.

Since we also ship the old name here and in 5.x upgrades will also work
with older flash-kernel.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
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librem5
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librem5
build:aarch64:Image

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