image.bbclass: Depend on virtual/kernel:do_deploy

Now that none of the packagegroups depend on virtual/kernel, we have the problem
that MACHINE=qemumips bitbake core-image-minimal doesn't put a kernel
into the deploy directory. This breaks many common usecases and
user expectations.

To avoid this, add a dependency on the kernel deploy to image do_build tasks.
This should avoid any circular dependency issues but equally ensure users
have their expectations met.

[YOCTO #5581]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>