Makefile: fix for pre-installed promu

When using BUILD_PROMU=false, this fixes the circular dependency
warning promu->promu, and avoids trying to download promu binary

Makefile.common

@@ -29,7 +29,7 @@ GO ?= go
 GOFMT ?= $(GO)fmt
 FIRST_GOPATH := $(firstword $(subst :, ,$(shell $(GO) env GOPATH)))
 ifeq ($(BUILD_PROMU),false)
- PROMU := promu
+ PROMU := $(shell which promu)
 else
  PROMU := $(FIRST_GOPATH)/bin/promu
 endif
@@ -199,11 +199,15 @@ common-docker-tag-latest:
 promu: $(PROMU)
 
 $(PROMU):
+ifeq ($(BUILD_PROMU),false)
+ @echo "using installed promu: $(PROMU)"
+else
  $(eval PROMU_TMP := $(shell mktemp -d))
  curl -s -L $(PROMU_URL) | tar -xvzf - -C $(PROMU_TMP)
  mkdir -p $(FIRST_GOPATH)/bin
  cp $(PROMU_TMP)/promu-$(PROMU_VERSION).$(GO_BUILD_PLATFORM)/promu $(FIRST_GOPATH)/bin/promu
  rm -r $(PROMU_TMP)
+endif
 
 .PHONY: proto
 proto: