Using the DTBs from the kernel snap rather than the gadget snap requires
an updated kernel snap and an updated snapd. The 'kernel-assets' feature
tells snapd to only install this (updated) kernel snap if an updated snapd
is present.
UBUNTU: [Packaging] Add KERNEL_SOURCE make variable
This is ignored by the actual make run but being in that domain
it is under our control. We will be using this to know which
kernel source really produces this snap. As with raspi2, there are
multiple sources that produce a pi-kernel (linux-raspi2 and
linux-raspi2-5.3 currently) that gets published to separate tracks.
Signed-off-by: Kleber Sacilotto de Souza <email address hidden>