refactor(mk): move build rules into separate scripts (#855)

See https://github.com/ooni/probe/issues/2218

script/copy-psiphon-files.bash

@@ -0,0 +1,20 @@
+#!/bin/bash
+set -euo pipefail
+
+if [[ $# -ne 1 ]]; then
+	echo "usage: $0 DIRECTORY" 1>&2
+	exit 1
+fi
+
+GIT_CLONE_DIR=$1
+mkdir -p $GIT_CLONE_DIR
+
+OONIPRIVATE_DIR=$GIT_CLONE_DIR/github.com/ooni/probe-private
+OONIPRIVATE_REPO=git@github.com:ooni/probe-private
+
+if [[ ! -d $OONIPRIVATE_DIR ]]; then
+	git clone $OONIPRIVATE_REPO $OONIPRIVATE_DIR
+fi
+
+cp $OONIPRIVATE_DIR/psiphon-config.key ./internal/engine
+cp $OONIPRIVATE_DIR/psiphon-config.json.age ./internal/engine