We don't need to append the dtb to the kernel image in all cases, with
e.g. the u-boot bootloader we can load the dtb seperately from the
kernel image. Introduce a new variable deviceinfo_append_dtb, if set to
"true", append the dtb, otherwise just copy the dtb file to the boot
partition.
Fixes#260
[ci:ignore-count]
* Added linux-postmarketos-allwinner kernel
* Added copy of alpines u-boot package with pine-a64lts support
* Changed postmarketos-mkinitfs to add the plain .dtb file to the boot partition