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 (closed)

The WIP items are:

• Some devices need the _mainline variable for the deviceinfo_dtb and deviceinfo_append_dtb variable now, as it's a hard fail now if the dtb file doesn't actually exist instead of just a warning.
• Change should be tested on the two Pine devices (should work fine though)
• The new variable has to be documented somewhere
• Maybe change the naming of the mkinitfs function as it now does something a bit different
• bump all the pkgrels