-modinst_dir = $(MODLIB)/$(if $(filter ../% /%,$@),extra/,kernel/$(@D))
+# Modules built outside the kernel source tree go into extra by default
+INSTALL_MOD_DIR ?= extra
+
+modinst_dir = $(MODLIB)/$(if $(filter ../% /%,$@),$(INSTALL_MOD_DIR)/,kernel/$(@D))