kernel: Turn on time-stamping (-N) for wget when getting kernel sources

wget downloads new linux-*.sign files and backs up the
existing files as file.1, file.2, etc. This causes false
positives during 'git status' or 'make check'
i.e.
* linux-2.6.38.3.tar.bz2.sign.1
* linux-3.2.7.tar.sign.1
Adding -N causes wget to smartly overwrite as required.

Makefile

@@ -54,8 +54,8 @@ get-sources: $(SRC_FILE)
 
 $(SRC_FILE):
 	@echo -n "Downloading $(URL)... "
-	@wget -q $(URL)
-	@wget -q $(URL_SIGN)
+	@wget -q -N $(URL)
+	@wget -q -N $(URL_SIGN)
 	@echo "OK."
 
 import-keys: