project_config_mk: fix if condition

make/project_config.mk

@@ -4,7 +4,7 @@
 COMPONENT_KCONFIGS := $(foreach component,$(COMPONENT_PATHS),$(wildcard $(component)/Kconfig))
 COMPONENT_KCONFIGS_PROJBUILD := $(foreach component,$(COMPONENT_PATHS),$(wildcard $(component)/Kconfig.projbuild))
 
-ifdef MSYSTEM
+ifeq ($(OS),Windows_NT)
 # kconfiglib requires Windows-style paths for kconfig files
 COMPONENT_KCONFIGS := $(shell cygpath -w $(COMPONENT_KCONFIGS))
 COMPONENT_KCONFIGS_PROJBUILD := $(shell cygpath -w $(COMPONENT_KCONFIGS_PROJBUILD))