circleci: Allow expanding Makefile in enterpise (#9587)

OSS no longer needs the multi-file config Makefile content, so it was
removed. However, enterprise does benefit from having that around. We
can allow enterprise to expand on OSS' CircleCI Makefile by adding an
include directive in OSS. This will allow the Makefile to be the same on
both OSS and Enterprise, so merge conflicts cannot occur.

.circleci/Makefile

@@ -27,3 +27,11 @@ help:
 .PHONY: ci-verify
 ci-verify:
 	@$(CIRCLECI) config validate config.yml
+
+MKFILE_PATH := $(lastword $(filter %Makefile,$(MAKEFILE_LIST)))
+CURRENT_DIR := $(dir $(realpath $(MKFILE_PATH)))
+CURRENT_DIR := $(CURRENT_DIR:/=)
+
+ifneq ($(wildcard $(CURRENT_DIR)/*.mk),)
+include $(CURRENT_DIR)/*.mk
+endif