Improve setup_dev_packages so that it can run incrementally.

BUG=none
TEST=ran to install latest libchrome, etc.

Change-Id: I7d48a77b1528f5fb4ea1ba5b7f52a9a248f71c84

Review URL: http://codereview.chromium.org/3169041
diff --git a/setup_dev_packages b/setup_dev_packages
index f7f162c..a8e6a7e 100755
--- a/setup_dev_packages
+++ b/setup_dev_packages
@@ -6,7 +6,7 @@
 
 set -ex
 
-sudo USE=-crash emerge -av1 \
+sudo USE=-crash emerge -DNauv1 \
   chromeos-base/libchrome \
   chromeos-base/metrics \
   dev-cpp/gmock \