Needs to ship librem-evk.dtb as well

Since the kernel sent upstream uses the same board name Purism Librem 5 devkit (and doesn't change to appending the board revision (as suggested)) but uses a different dtb name (imx8mq-librem5-devkit.dtb) we need to ship a dtb with the above name as well in order not break kernel installations.

I had a look at flash-kernel if there are ways around this but didn't spot anything. Other workarounds like dumping files in /etc/flash-kernel are even more cumbersome to maintain.