setup-environment: Move DL_DIR setting to local.conf

This improves user-friendness as it makes clear where it is storing
the downloads cache.

Change-Id: Iedd2d22b938ea192dda96ef2c80a716d6796e53f
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
setup-environment: Allow user to provide a site.conf

The site.conf file is intended to provide configuration which is used
by default in the site where it is building.

There are several uses as:

- override DL_DIR for a company shared cache;
- set proxy value for GIT;
- etc...

Change-Id: I36bf6c5e1c192391b996a49f54cd4f413c769cb0
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>