small doc update

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/INSTALL b/INSTALL
index 2d645cd..ec2b028 100644
--- a/INSTALL
+++ b/INSTALL
@@ -117,7 +117,7 @@
 
 To build out of tree, cd to an empty directory and configure busybox from there:
 
-  make -f /path/to/source/Makefile defconfig
+  make KBUILD_SRC=/path/to/source -f /path/to/source/Makefile defconfig
   make
   make install