Implement package build, push and show-tag in linuxkit tool

This implements the proposal in #2564 and converts a handful of representative
or especially interesting (from a build PoV) packages to use it.

For now those pkg/* affected get a stub-`Makefile`, once all packages are
converted then `pkg/Makefile` can be adjusted and those stubs can be removed.

For now only `pkg/package.mk`'s functionality is implemented. In particular:

- `push-manifest.sh` remains a separate script, to enable calling it on systems
  with just the LinuxKit tools installed arrange to install it under a less
  generic name.
- `kernel` and `tools/alpine` do not use `pkg/package.mk` and those cases are
  not yet fully considered/covered.

I have updated the documentation assuming that the existing uses of
`pkg/package.mk` will be removed quite soon in a follow up PR rather than
trying to document the situation which results after just this commit.

Due to `cmd/linuxkit` now gaining a library the build needs adjusting slightly to
allow both `make bin/linuxkit` and `go build` to work.

`go vet` has forced me to write some rather asinine comments for things that
are rather obvious from the name.

Signed-off-by: Ian Campbell <ijc@docker.com>

projects/kubernetes/image-cache/Makefile.pkg

@@ -1,11 +1,22 @@
-ORG?=linuxkitprojects
-IMAGE=kubernetes-image-cache-$(CACHE)
-NOTRUST=1
-SOURCE=$(BUILDDIR)
-DEPS=$(BUILDDIR)/Dockerfile
-ARCHES=x86_64
+$(BUILDDIR)/build.yml: build.yml.in
+	@sed -e 's/@@CACHE@@/$(CACHE)/g' < $< > $@
 
 $(BUILDDIR)/Dockerfile: Dockerfile
-	cp $< $@
+	@cp $< $@
 
-include ../../../pkg/package.mk
+.PHONY: push forcepush tag forcetag show-tag
+
+push: $(BUILDDIR)/build.yml $(BUILDDIR)/Dockerfile
+	linuxkit pkg push -hash-path . $(BUILDDIR)
+
+forcepush: $(BUILDDIR)/build.yml $(BUILDDIR)/Dockerfile
+	linuxkit pkg push -force -hash-path . $(BUILDDIR)
+
+tag: $(BUILDDIR)/build.yml $(BUILDDIR)/Dockerfile
+	linuxkit pkg build -hash-path . $(BUILDDIR)
+
+forcetag: $(BUILDDIR)/build.yml $(BUILDDIR)/Dockerfile
+	linuxkit pkg build --force -hash-path . $(BUILDDIR)
+
+show-tag: $(BUILDDIR)/build.yml $(BUILDDIR)/Dockerfile
+	@linuxkit pkg show-tag -hash-path . $(BUILDDIR)

projects/kubernetes/image-cache/build.yml.in

@@ -0,0 +1,5 @@
+org: linuxkitprojects
+image: kubernetes-image-cache-@@CACHE@@
+trust: false
+arches:
+  - amd64