Retrieve directory list from environment variables

Retrieve directory list from environment variables

This adds a function able to retrieve from a prefix and suffix
an environment variable. This is meant to search config or plugin
files from potentially several sources, environment and source code.

This tries to get 2 environments variables:

- prefix+binderName+suffix (eg: AFT_LOWCAN_CONFIG_PATH)
- prefix+suffix (eg: AFT_CONFIG_PATH)

Then it returns the one found or both if they exist with the
most accurate one (with the binder name) first followed by the
other.

Change-Id: Ic448ff017e6158bec05895d63688b8968b5c6434
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>