Based on some new (to me...) info from Purism's Sebastian K, there are
apparently Evergreen devices that return different values for board rev,
so we shouldn't assume only r4 are Evergreen, and default to the r2 DT.
But many earlier devices (Birch, Chestnut, Dogwood) may or may not have
the board rev set either. Who knows! Defaulting to the Evergreen DT on
these devices is bad too, but there weren't many of those shipped (I
guess?), at least not compared to the number of Evergreens that aren't
properly identifiable in u-boot... so this using this DT by default is
the best of the two bad options we have.
This comment thread has more info:
https://gitlab.com/postmarketOS/pmaports/-/issues/1643#note_1147248594fixes#1643
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