Makefile: Add a .PHONY target for the opm build target (#813)

Update the root Makefile and add a .PHONY target for the bin/opm target.
This is needed to as the bin/opm target outputs the opm executable in
the same name as the target, which is problematic in the case there's an
existing ./bin/opm file present and a re-build is necessary.

Signed-off-by: timflannagan <[email protected]>
Upstream-repository: operator-registry
Upstream-commit: 3e01e6cccf8ed1a26ae2ee0c922250854949db5e

Author: timflannagan

staging/operator-registry/Makefile

@@ -37,6 +37,7 @@ all: clean test build
 $(CMDS):
 	$(GO) build $(extra_flags) $(TAGS) -o $@ ./cmd/$(notdir $@)
 
+.PHONY: $(OPM)
 $(OPM): opm_version_flags=-ldflags "-X '$(PKG)/cmd/opm/version.gitCommit=$(GIT_COMMIT)' -X '$(PKG)/cmd/opm/version.opmVersion=$(OPM_VERSION)' -X '$(PKG)/cmd/opm/version.buildDate=$(BUILD_DATE)'"
 $(OPM):
 	$(GO) build $(opm_version_flags) $(extra_flags) $(TAGS) -o $@ ./cmd/$(notdir $@)