This is in preparation for the modeenv to merely track what list of kernels are valid to have booted from. For now, we shouldn't be using the Kernel setting in the modeenv at all.
Note that while we don't use the modeenv in snap-bootstrap in snapd master, it might still take a while for the ubuntu-core-initramfs package and thus the kernel snap to catch-up with the changes from #8001. This should only be a problem temporarily until I can get #8069 working (which is hopefully soon).
As implemented the slot can be either statically defined by the gadget
snap, where it must contain a path attribute that points to
/dev/uio[0-9]+ device name.
No hotplug support included yet.
I've successfully tested the interface on a customer device.
prepare.sh: download w/basename + use as arg to uc20_build_initramfs_kernel_snap
This prevents conflicts with pc-kernel_389.snap (revision name from snap
download) and pc-kernel_5.4.3.12_amd64.snap (version + arch from snap pack) if
we ever happen to be running in the same directory.