Use flash-image to install the dev kit kernel
Installing flash-kernel/scratch specifically to avoid the upstream version which does not support this device.
Once devkit support lands in upstream flash-kernel, the regular version can be installed.
Note - I left both flash-kernel/scratch and u-boot-tools in the board file, because I'm not certain the /scratch version (librem5 branch) can be selected as part of librem5-base.