mk: in install rule, don't overwrite .config if it already exists

This allows the user to prepare a configuration with make config
before using make install.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>

mk/rte.sdkinstall.mk

@@ -56,7 +56,9 @@ install: $(INSTALL_TARGETS)
 
 %_install:
 	@echo ================== Installing $*
-	$(Q)$(MAKE) config T=$* O=$(BUILD_DIR)/$*
+	$(Q)if [ ! -f $(BUILD_DIR)/$*/.config ]; then \
+		$(MAKE) config T=$* O=$(BUILD_DIR)/$*; \
+	fi
 	$(Q)$(MAKE) all O=$(BUILD_DIR)/$*
 
 #