Makefile: Add a pattern rule for oci-* commands

Make it easy to (re)build a single command without building all of
them.  Using main.go as the only prerequisite cuts some corners, but
prerequisites for this are hard ;).  With this commit, you can touch
main.go to force a rebuild.  If that ends up being too awkward, we can
make these .PHONY targets.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

Makefile

@@ -2,6 +2,10 @@ GO15VENDOREXPERIMENT=1
 export GO15VENDOREXPERIMENT
 
 EPOCH_TEST_COMMIT ?= v0.2.0
+TOOLS := \
+	oci-create-runtime-bundle \
+	oci-image-validate \
+	oci-unpack
 
 default: help
 
@@ -18,10 +22,10 @@ check-license:
 	@echo "checking license headers"
 	@./.tool/check-license
 
-tools:
-	go build ./cmd/oci-create-runtime-bundle
-	go build ./cmd/oci-unpack
-	go build ./cmd/oci-image-validate
+tools: $(TOOLS)
+
+oci-%: cmd/oci-%/main.go
+	go build ./cmd/$@
 
 lint:
 	@echo "checking lint"
@@ -67,10 +71,7 @@ install.tools: .install.gitvalidation .install.glide .install.glide-vc .install.
 	gometalinter --install --update
 
 clean:
-	rm -rf *~ $(OUTPUT_DIRNAME)
-	rm -f oci-create-runtime-bundle
-	rm -f oci-unpack
-	rm -f oci-image-validate
+	rm -rf *~ $(OUTPUT_DIRNAME) $(TOOLS)
 
 .PHONY: \
 	tools \