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

src/scripts/common.sh

Index: src/scripts/common.sh
diff --git a/src/scripts/common.sh b/src/scripts/common.sh
index fc7a897d61346f2a40426930e6f37dba70a3b9fa..25dee4a3ed6c634b6d287689e539a83ea97db76a 100644
--- a/src/scripts/common.sh
+++ b/src/scripts/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".