setup_kernel: Don't setup kernel on devkit manually

We handle this via librem5-devkit-base to make the installer more slime
and have less cruft in the image builder.

This way we already create an initramfs which can later on be used by
flash-kernel.

Closes: #23
1 job for <span class="ref-name">setup-kernel</span> in 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #6626
librem5
job

00:00:14