build: fix ordering of pkgbase targets to get a dtb package reliably

dtb.plist is generated by mtree-to-plist when it processes the kernel
metalog, so we shouldn't try to create dtb packages until that's been
done. Add another level of indirection to the create-kernel-packages
target to organize all of the kernel flavors under a single target that
is ordered before the create-dtb-packages target.

Note that we may later refine these targets after further pkgbase
review; this is simply a maybe-final stop-gap that's been tested to
fix a known problem.

MFC after: 37 seconds (pkgbase fix for 15.0 builds)
Reviewed and tested by: cperciva
Differential Revision: https://reviews.freebsd.org/D53017