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>

pkg/binfmt/Makefile

@@ -1,5 +1,16 @@
-IMAGE=binfmt
-DEPS=main.go $(wildcard etc/binmft.d/*)
-ARCHES=x86_64
+.PHONY: push forcepush tag forcetag show-tag
 
-include ../package.mk
+push:
+	linuxkit pkg push .
+
+forcepush:
+	linuxkit pkg push --force .
+
+tag:
+	linuxkit pkg build .
+
+forcetag:
+	linuxkit pkg build --force .
+
+show-tag:
+	linuxkit pkg show-tag .

pkg/binfmt/build.yml

@@ -0,0 +1,3 @@
+image: binfmt
+arches:
+  - amd64