How to pass environment variables to packer in AWS?

Question

I am building a custom AMI in AWS using packer and bash + salt provisioning. However, I am in need of being able to pass some variables from my local environment to the build system and I don't really know how to do that when building using "ebp" ("eb platform" of awsebcli).

My packer json contains stuff like this:

"variables": {
    "platform_name": "{{env `AWS_EB_PLATFORM_NAME`}}",
    "platform_version": "{{env `AWS_EB_PLATFORM_VERSION`}}",
    "platform_arn": "{{env `AWS_EB_PLATFORM_ARN`}}",
    "build_app": "{{env `MY_BUILD_APP_NAME`}}",
    "build_env": "{{env `MY_BUILD_ENV_NAME`}}"
 },

and later:

"provisioners": [
    {
      "type": "file",
      "source": "builder",
      "destination": "/tmp/"
    },
    {
       "type": "shell",
      "execute_command": "chmod +x {{ .Path }}; {{ .Vars }} sudo {{ .Path }}",
      "scripts": [
         "builder/builder.sh"
      ],
      "environment_vars": [
          "AMI_BUILD_APP={{user `build_app`}}",
          "AMI_BUILD_ENV={{user `build_env`}}"

I know that if I were to run packer locally, that would be enough. However, I only run "ebp create" and packer runs on the remote build environment in EC2.

Where do I need to define MY_BUILD_APP_NAME and MY_BUILD_ENV_NAME so that the packer buider environment will make them available to the bash provisioner?

Thanks!