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.
This isn't ideal but it's better to switch flash-kernel know than later. The fact that it's mostly includes makes this zero maintenance.
Signed-off-by: Guido Günther email@example.com