Add a dependency on the kernel package for the runtime package.

The idea here is that, provided upstream pkg(8) maintainers accept
the proposed change, the kernel.ucl will contain a post-install
script causing pkg(8) to emit a message informing to reboot the
system after the kernel is upgraded using 'pkg upgrade', so the
new userland is installed on the running new kernel. At present,
this functionality does not exist in pkg(8), but will help ensure
the upgrade path follows that from UPDATING. To work around this
for now, evaluate ASSUME_ALWAYS_YES, and prompt the user if they
wish to proceed if not set to true.

Since there is a kernel dependency, and a non-GENERIC kernel may
be in use, update Makefile.inc1 to replace '%KERNCONF%' in the
runtime.ucl with the first-built kernel set either via command line
or in make.conf(5).

MFC after: 5 days
Sponsored by: The FreeBSD Foundation