extensions: adapt for new rpm-ostree support

With coreos/rpm-ostree#2439 and
coreos/coreos-assembler#2028, support for
extensions is now more robust.

The extensions.yaml file is now fed directly to rpm-ostree. We just need
to do some minor tweaks:
- drop listing usbguard deps, rpm-ostree fetches them automatically
- mark `kernel-devel` as a development extension; this will signal the
  MCO that it's not intended to be directly layered on the host
- use `match-base-evr: kernel` to enforce that the kernel development
  packages have the same EVR as the base kernel package in the OSTree

extensions.yaml

@@ -1,26 +1,26 @@
 # RPMs as operating system extensions, distinct from the base ostree commit/image
 # https://github.com/openshift/enhancements/blob/master/enhancements/rhcos/extensions.md
 # and https://github.com/coreos/fedora-coreos-tracker/issues/401
-# We currently explicitly list dependencies for each extension; see
-# https://github.com/coreos/rpm-ostree/issues/2055
+
+repos:
+  - rhel-8-nfv
 
 extensions:
   # https://github.com/coreos/fedora-coreos-tracker/issues/326
   usbguard:
     packages:
       - usbguard
-      - libqb
-      - protobuf
-  # kernel-devel
   # https://github.com/kmods-via-containers/kmods-via-containers/issues/3
   # https://gitlab.cee.redhat.com/coreos/redhat-coreos/merge_requests/866
   kernel-devel:
+    kind: development
     packages:
       - kernel-devel
       - kernel-core
       - kernel-headers
       - kernel-modules
       - kernel-modules-extra
+    match-base-evr: kernel
   # GRPA-2822
   # https://github.com/openshift/machine-config-operator/pull/1330
   # https://github.com/openshift/enhancements/blob/master/enhancements/support-for-realtime-kernel.md