Makefile: remove hard setting CROSS_COMPILE

* linux-2.6.16-chumby-1.2.tar.gz set CROSS_COMPILE as arm-linux- hardly
* It causes the AR used to compiling linux kernel is arm-linux-ar, which doesn't exist in openembedded at all
Index: linux-2.6.16/Makefile
===================================================================
--- linux-2.6.16.orig/Makefile	2010-02-01 16:19:36.000000000 +0800
+++ linux-2.6.16/Makefile	2010-02-01 16:20:02.000000000 +0800
@@ -173,8 +173,7 @@
 # Note: Some architectures assign CROSS_COMPILE in their arch/*/Makefile
 
 ARCH		?= $(SUBARCH)
-#CROSS_COMPILE	?=
-CROSS_COMPILE = arm-linux-
+CROSS_COMPILE	?=
 
 # Architecture as present in compile.h
 UTS_MACHINE := $(ARCH)