[PATCH] fix mips/Makefile to support CROSS_COMPILE from environment var

[Tim Bird <tim.bird@am.sony.com>]

The following patch allows to build using CROSS_COMPILE from the environment.
I have an automated system which works like this, but it chokes on MIPS when
I use it with (albeit non-standard-named) cross-compiler tools.  An easy
workaround I'm using is to put CROSS_COMPILE on the make command line, but it would
be nice to use the definition already in the environment when I work
manually in the system.

For past discussion of this see:
http://www.linux-mips.org/archives/linux-mips/2003-02/msg00196.html

I'm not sure why the change didn't make it in back in 2003, but
if the complaint was about the use of "?=", that seems to be in use
other places, and fairly standard now.

For example, from the top level kernel Makefile:
ARCH            ?= $(SUBARCH)
CROSS_COMPILE   ?=

Signed-off-by: Tim Bird <tim.bird@am.sony.com>

---
--- alp-linux.orig/arch/mips/Makefile	2006-05-04 12:22:17.000000000 -0700
+++ alp-linux/arch/mips/Makefile	2006-05-04 12:10:22.000000000 -0700
@@ -49,7 +49,7 @@ UTS_MACHINE		:= mips64
 endif

 ifdef CONFIG_CROSSCOMPILE
-CROSS_COMPILE		= $(tool-prefix)
+CROSS_COMPILE		?= $(tool-prefix)
 endif

 CHECKFLAGS-y				+= -D__linux__ -D__mips__ \