Under Cygwin, executable has a different name

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@281 42af7a65-404d-4744-a932-0658087f49c3

tools/Makefile.mkconfig

@@ -40,4 +40,4 @@ mkconfig: mkconfig.c
 	gcc -O2 -Wall -o mkconfig mkconfig.c
 
 clean:
-	rm -f mkconfig *~
+	@rm -f mkconfig mkconfig.exe *~