This commit changes the supported yaml for the kernel snap. For
the raspberry pi DTB use case the kernel assets are tightly
coupled with the kernel. So the edition would have to be bumped
everytime the kernel is build. So the edition does not make much
sense in this context. Hence a new "update" field that is boolean
for now but we may expand it later into a map. This map would
allow to specify what content items should get updated and which
should not get updated.
This commit moves the kernel.yaml parsing into the kernel package
and extracts the gadget:editionNumber" helper into it's own
gadget/editon packages so that it can be shared between the
kernel and the gadget packages. Because meta/kernel.yaml is
optional it is not an error if it is missing.
Extra tests are also added.
