prepare: Unset make environment variables

These interfere with building/installing packages with Portage.

prepare.sh

@@ -5,6 +5,8 @@ set -e
 
 . "${CONFIGDIR:=${PWD}}"/config
 
+unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
+
 # Ensure we have a Portage repo
 if [ ! -f /var/db/repos/gentoo/metadata/timestamp ]; then
     emerge-webrsync