pre-processor only works with relative path based on input file with the complete path

[GitHub issues - opened]

alsa-project/alsa-utils issue #123 was opened from ranj063:

For ex:

`alsatplg -P sof-hda-generic.conf -o test.conf` fails with:

Unable to open included conf file sof-hda-generic.conf/include/components/eqiir/highpass_40hz_0db_48khz.conf
Unable to process include file 
Failed to process includes
Failed to process conditional includes in input config

whereas:

`alsatplg -P ~/work/sof/sof/tools/topology/topology2/sof-hda-generic.conf -o test.conf` works OK.

Would it be possible to make the first command work with the use of the ALSA_CONFIG_DIR environment variable?

Issue URL     : https://github.com/alsa-project/alsa-utils/issues/123
Repository URL: https://github.com/alsa-project/alsa-utils