Use fileSeparator to concat /etc

Use fileSeparator to concat /etc

On windows we have a different file separator, this is probably
causing:

[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.opendaylight.odlparent:karaf-plugin:6.0.0:populate-local-repo (populate-local-repo) on project distribution.opendaylight-karaf: Failed to execute: Could not resolve URI: file:D:\Open-Day-Light\magnesium-mri\controller\karaf\target\assembly/etc/3099bac6-f5e3-4ba5-81f1-de70eb9fc227.xml: Illegal character in opaque part at index 7: file:D:\Open-Day-Light\magnesium-mri\controller\karaf\target\assembly/etc/3099bac6-f5e3-4ba5-81f1-de70eb9fc227.xml -> [Help 1]

Change-Id: Id264d4bb0e15cde47da6913996d173e9bf5ea03e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>