Fixes for attribute value validation in the topology pre-processor

[GitHub pull_request - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-utils pull request #138 was opened from ranj063:

This PR fixes 2 issues:
1. Allow variable definitions within included conf files
2. Validating attributes after expanding the variables

Request URL   : https://github.com/alsa-project/alsa-utils/pull/138
Patch URL     : https://github.com/alsa-project/alsa-utils/pull/138.patch
Repository URL: https://github.com/alsa-project/alsa-utils