Make mkinitfs fail harder when the dtb is missing on mainline kernels
See @z3ntu's note at 63f921fb (comment 101525375)
Currently postmarketos-mkinitfs only shows a NOTE: in the build when the specified .dtb for the device can't be found. It currently doesn't trow an fatal exception since that would break devices that have both an upstream and downstream kernel.
It would be better if postmarketos-mkinitfs did throw an fatal error on missing .dtb files only if the current kernel is an upstream kernel so downstream kernels still would work and upstream kernel development is slightly easier.
I don't see a quick way to see which folder contained the original kernel package from the mkinitfs script so it's probably easiest to only show the error if the initramfs name contains postmarketos