build: Make enter_chroot.sh fail if run as root.

BUG=684

Review URL: http://codereview.chromium.org/429006

git-svn-id: svn://chrome-svn/chromeos/trunk@304 06c00378-0e64-4dae-be16-12b19f9950a1

common.sh

@@ -171,6 +171,13 @@ function assert_outside_chroot {
   fi
 }
 
+function assert_not_root_user {
+  if [ `id -u` = 0 ]; then
+    echo "This script must be run as a non-root user."
+    exit 1
+  fi
+}
+
 # Install a package if it's not already installed
 function install_if_missing {
   # Positional parameters from calling script.  :? means "fail if unset".

enter_chroot.sh

@@ -10,8 +10,9 @@
 # The path to common.sh should be relative to your script's location.
 . "$(dirname "$0")/common.sh"
 
-# Script must be run outside the chroot
+# Script must be run outside the chroot and as a regular user.
 assert_outside_chroot
+assert_not_root_user
 
 # Define command line flags
 # See http://code.google.com/p/shflags/wiki/Documentation10x