ISH: set toolchain in core/minute-ia

When "make buildall" has board which uses core/minute-ia,
CROSS_COMPILE?=i686-pc-linux-gnu- doesn't get set unless
CROSS_COMPILE is not defined; however, it's defined before
this line, and wrong toolchain is used.
Remove "?" to set correct CROSS_COMPILE.

BUG=none
BRANCH=none
TEST='make buildall -j' passed

Change-Id: Ied4a9f93a4d44714c012d3a3e50e4a34f41a7c1f
Signed-off-by: li feng <li1.feng@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/410402
Commit-Ready: Li1 Feng <li1.feng@intel.com>
Tested-by: Li1 Feng <li1.feng@intel.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

core/minute-ia/build.mk

@@ -1,5 +1,5 @@
 # -*- makefile -*-
-# Copyright (c) 2016 The Chromium OS Authors. All rights reserved.
+# Copyright 2016 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
@@ -7,7 +7,7 @@
 #
 
 # Select Minute-IA bare-metal toolchain
-CROSS_COMPILE?=i686-pc-linux-gnu-
+CROSS_COMPILE=i686-pc-linux-gnu-
 
 # FPU compilation flags
 CFLAGS_FPU-$(CONFIG_FPU)=
@@ -15,7 +15,7 @@ CFLAGS_FPU-$(CONFIG_FPU)=
 # CPU specific compilation flags
 CFLAGS_CPU+=-fno-omit-frame-pointer -mno-accumulate-outgoing-args	\
 	    -ffunction-sections -fdata-sections				\
-	    -fno-builtin-printf -fno-builtin-sprintf	 		\
+	    -fno-builtin-printf -fno-builtin-sprintf			\
 	    -fno-stack-protector -gdwarf-2  -fno-common -ffreestanding	\
 	    -minline-all-stringops -fno-strict-aliasing