build: only touch .dir.stamp if it doesn't already exist

This stops its date being continuously updated, forcing configure
retests all the time.  Now it serves its intended purpose of causing
configure retests only if .config was deleted (as is done by e.g.
make clean).

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

Makeconfig

@@ -132,8 +132,10 @@ $(CONFIG_MK): $(objdir)/.config/config.$(1).mk
 endef
 
 $(objdir)/.config/.dir.stamp:
-	mkdir -p $(objdir)/.config
-	touch $(objdir)/.config/.dir.stamp
+	if [[ ! -f $(objdir)/.config/.dir.stamp ]]; then \
+	    mkdir -p $(objdir)/.config; \
+	    touch $(objdir)/.config/.dir.stamp; \
+	fi
 
 $(CONFIG_H):
 	echo '/* This file is automatically generated. */' > $(objdir)/config.h