Add additional ldflags constants for Kubernetes versioning

Now it is possible to push these variables through ldflags:
- gitMajor
- gitMinor
- gitVersion (from latest annotated tag, output of git describe)
- gitCommit (renamed commitFromGit, intended to have the full sha1)
- gitTreeState (either "clean" or "dirty")

These are spawned into its separate source file, since they are meant to
be updated separately when a new version is released.

Also use the notation vX.Y+ for when git information is not present.
(This is consistent with the kernel build, e.g. Linux 3.15+ means its
version is >= 3.15 and < 3.16.)

v2: Added comments to the individual fields in pkg/version/base.go

Tested:
- Built it and checked the -version output:
  $ hack/build-go.sh
  $ output/go/bin/kubelet -version
  Kubernetes version 0.1+, build c328679ef8aa

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

pkg/version/version.go

@@ -20,26 +20,28 @@ import (
 	"fmt"
 )
 
-// commitFromGit is a constant representing the source version that
-// generated this build. It should be set during build via -ldflags.
-var commitFromGit string
-
 // Info contains versioning information.
 // TODO: Add []string of api versions supported? It's still unclear
 // how we'll want to distribute that information.
 type Info struct {
-	Major     string `json:"major" yaml:"major"`
-	Minor     string `json:"minor" yaml:"minor"`
-	GitCommit string `json:"gitCommit" yaml:"gitCommit"`
+	Major        string `json:"major" yaml:"major"`
+	Minor        string `json:"minor" yaml:"minor"`
+	GitVersion   string `json:"gitVersion" yaml:"gitVersion"`
+	GitCommit    string `json:"gitCommit" yaml:"gitCommit"`
+	GitTreeState string `json:"gitTreeState" yaml:"gitTreeState"`
 }
 
 // Get returns the overall codebase version. It's for detecting
 // what code a binary was built from.
 func Get() Info {
+	// These variables typically come from -ldflags settings and in
+	// their absence fallback to the settings in pkg/version/base.go
 	return Info{
-		Major:     "0",
-		Minor:     "1",
-		GitCommit: commitFromGit,
+		Major:        gitMajor,
+		Minor:        gitMinor,
+		GitVersion:   gitVersion,
+		GitCommit:    gitCommit,
+		GitTreeState: gitTreeState,
 	}
 }