* kernel,overlord: use function from kernel pkg to get kernel early
mount dir.
* o/snapstate/sequence: add ancillary methods to retrieve components
for a given snap revision in the sequence. One of the methods can be
given a component type for filtering.
* o/snapstate/backend: add methods to set-up kernel-modules components
* o/snapstate: add handlers to set-up kernel-modules components
* o/snapstate: add specific setup task for kernel-modules components
* o/{devicestate,snapstate}: change name of kernel set-up tasks
* systemd: add before dependency to early mounts for local-fs.target
So dependencies are the same as when mounting in /snap, as that is
where we will do also the early mounts now.
* kernel: use normal mount points when building drivers tree
* overlord/devicestate: no need to remove early mount anymore
* overlord/snapstate: consider kernel type when ensuring the mounts
* kernel: put modules back in "updates" folder
As that gives priority to the kernel modules under that directory (see
depmod.d(5)).
And ancillary methods:
- KernelVersionFromPlaceInfo, to find the kernel version ("uname -r")
for a mounted snap.
- BuildKernelDriversTree, to build a tree from a mounted snap that can
be bind-mounted to /usr/lib/{modules,firmware}.
These methods are needed to support kernel-modules components.
