build: Check for .git, but it doesn't have to be a directory, so that esp-idf will work as a submodule

Merges #438 https://github.com/espressif/esp-idf/pull/438

make/project.mk

@@ -409,7 +409,7 @@ define GenerateSubmoduleCheckTarget
 check-submodules: $(IDF_PATH)/$(1)/.git
 $(IDF_PATH)/$(1)/.git:
 	@echo "WARNING: Missing submodule $(1)..."
-	[ -d ${IDF_PATH}/.git ] || ( echo "ERROR: esp-idf must be cloned from git to work."; exit 1)
+	[ -e ${IDF_PATH}/.git ] || ( echo "ERROR: esp-idf must be cloned from git to work."; exit 1)
 	[ -x $(which git) ] || ( echo "ERROR: Need to run 'git submodule init $(1)' in esp-idf root directory."; exit 1)
 	@echo "Attempting 'git submodule update --init $(1)' in esp-idf root directory..."
 	cd ${IDF_PATH} && git submodule update --init $(1)