disable color in emerge output

If the default settings enable color, then this parsing fails.

BUG=None
TEST=build_image still works when emerge defaults to color output

Change-Id: I3c5be726d22362cd5b96e3bb89327525e0194e48
Reviewed-on: https://chromium-review.googlesource.com/382791
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
diff --git a/build_library/base_image_util.sh b/build_library/base_image_util.sh
index 17738cc..d2dcca1 100755
--- a/build_library/base_image_util.sh
+++ b/build_library/base_image_util.sh
@@ -62,7 +62,7 @@
   local pkgs_out=$(mktemp -d)
 
   for pkg in "${pkgs[@]}" ; do
-    emerge-${BOARD} --pretend --quiet --emptytree \
+    emerge-${BOARD} --color n --pretend --quiet --emptytree \
       --root-deps=rdeps ${pkg} | \
       egrep -o ' [[:alnum:]-]+/[^[:space:]/]+\b' | \
       tr -d ' ' | \