Makefile: Print diff for BPF skeleton files

I was hitting an issue where I would run 'make' and it would just fail
out like:

    $ make V=0
    ...
      GEN    bpf/../pkg/datapath/types
      GEN    bpf/../pkg/datapath/config
      GEN    bpf/../pkg/datapath/maps
      GEN    bpf/../pkg/datapath/bpf
    make: Leaving directory '/go/src/github.com/cilium/cilium/bpf'
    c706c5a9620a7cd97409c69ee6e052b61e9094c1c8278d69834285ddd7dd1d52
    please run 'make generate-bpf' and submit your changes
    make: *** [Makefile:510: precheck] Error 1

But if I ran make generate-bpf, it wouldn't generate a diff:

    $ make generate-bpf V=0
      DOCKER  generate-bpf
    7d9a426f066e8a7effd17434d31c3818583797488e75b33ef30b42f8ea4c08a7
    make: Entering directory '/go/src/github.com/cilium/cilium/bpf'
      GEN    bpf/../pkg/datapath/types
      GEN    bpf/../pkg/datapath/config
      GEN    bpf/../pkg/datapath/maps
      GEN    bpf/../pkg/datapath/bpf
    make: Leaving directory '/go/src/github.com/cilium/cilium/bpf'
    7d9a426f066e8a7effd17434d31c3818583797488e75b33ef30b42f8ea4c08a7
    $ git diff
    $ git diff --cached
    $

Improve the debuggability by always doing a check and echoing the files
that may conflict. Technically this should maybe be tied into the script
'./contrib/scripts/check-datapathconfig.sh', but by putting it in this
target we can retain the consistent quiet make output without adding
extra custom logic into that script.

Result:

    $ make generate-bpf
      DOCKER  generate-bpf
    6cad562da3cf20b0a38092a5ddb02a5f45b426a8e4f1c55658628bb580c6eac1
    make: Entering directory '/go/src/github.com/cilium/cilium/bpf'
      GEN    bpf/../pkg/datapath/types
      GEN    bpf/../pkg/datapath/config
      GEN    bpf/../pkg/datapath/maps
      GEN    bpf/../pkg/datapath/bpf
    make: Leaving directory '/go/src/github.com/cilium/cilium/bpf'
    6cad562da3cf20b0a38092a5ddb02a5f45b426a8e4f1c55658628bb580c6eac1
      CHECK   bpf-skel
    ?? pkg/my-dirty-file

Signed-off-by: Joe Stringer <joe@cilium.io>

Author: joestringer

Makefile

@@ -405,6 +405,8 @@ generate-bpf: ## Generate config structs from BPF objects using dpgen and Go ske
 	@$(ECHO_DOCKER)
 	contrib/scripts/builder.sh \
 		$(MAKE_CONTAINER) -C /go/src/github.com/cilium/cilium/bpf generate V=$(V)
+	@$(ECHO_CHECK) bpf-skel
+	$(QUIET)git status bpf/ pkg/ --porcelain
 
 check-k8s-clusterrole: ## Ensures there is no diff between preflight's clusterrole and runtime's clusterrole.
 	./contrib/scripts/check-preflight-clusterrole.sh