main/postmarketos-mkinitfs: Use the original filename for the dtb

See !528 (comment 196796903)

Looking at the contents of the rootfs chroot, it should be fine but it should still be tested on at least one device.

@craftyguy Not really related here, but for the Librem 5, imx8mq-librem5-devkit.dtb should be used, as indicated by https://source.puri.sm/Librem5/linux-next/commit/b26766d2adad0423efd88dc28f194db7de624bb9 (and a few other issues in the repo)