Makefile target to build a single plugin for local testing.

Often when developing a plugin it's only necessary to rebuild that plugin.
Here we add a simple Makefile target that makes that easy:

    make plugin-dev PLUGIN=provider-aws

Since it's only building one package and it's only building for the
host architecture, this just uses "go install" directly, rather than using
gox as we do when installing multiple packages, possibly for multiple
architectures.

CHANGELOG.md

@@ -71,6 +71,7 @@ BUG FIXES:
 
 INTERNAL IMPROVEMENTS:
 
+  * core: Makefile target "plugin-dev" for building just one plugin. [GH-3229]
   * helper/schema: Don't allow ``Update`` func if no attributes can actually be updated, per schema. [GH-3288]
   * helper/schema: Default hashing function for sets [GH-3018]
   * helper/multierror: Remove in favor of [github.com/hashicorp/go-multierror](http://github.com/hashicorp/go-multierror). [GH-3336]

Makefile

@@ -15,6 +15,12 @@ dev: generate
 quickdev: generate
 	@TF_QUICKDEV=1 TF_DEV=1 sh -c "'$(CURDIR)/scripts/build.sh'"
 
+# Shorthand for building and installing just one plugin for local testing.
+# Run as (for example): make plugin-dev PLUGIN=provider-aws
+plugin-dev: generate
+	go install github.com/hashicorp/terraform/builtin/bins/$(PLUGIN)
+	mv $(GOPATH)/bin/$(PLUGIN) $(GOPATH)/bin/terraform-$(PLUGIN)
+
 release: updatedeps
 	gox -build-toolchain
 	@$(MAKE) bin