Makefile: add make swarm command (#14698)

* Makefile: add make swarm command

* Makefile: minor code formatting polishes

Makefile

@@ -2,7 +2,7 @@
 # with Go source code. If you know what GOPATH is then you probably
 # don't need to bother with make.
 
-.PHONY: geth android ios geth-cross evm all test clean
+.PHONY: geth android ios geth-cross swarm evm all test clean
 .PHONY: geth-linux geth-linux-386 geth-linux-amd64 geth-linux-mips64 geth-linux-mips64le
 .PHONY: geth-linux-arm geth-linux-arm-5 geth-linux-arm-6 geth-linux-arm-7 geth-linux-arm64
 .PHONY: geth-darwin geth-darwin-386 geth-darwin-amd64
@@ -16,6 +16,11 @@ geth:
 	@echo "Done building."
 	@echo "Run \"$(GOBIN)/geth\" to launch geth."
 
+swarm:
+	build/env.sh go run build/ci.go install ./cmd/swarm
+	@echo "Done building."
+	@echo "Run \"$(GOBIN)/swarm\" to launch swarm."
+
 evm:
 	build/env.sh go run build/ci.go install ./cmd/evm
 	@echo "Done building."