Set the FreeBSD /etc/salt dir to PREFIX/etc/salt

The FreeBSD port version of saltstack uses /usr/local/etc/salt rather
than /etc/salt for its configuration.
Respect the environment.

Remove the check for the TEMP_CONFIG_DIR as the FreeBSD SALT_ETC_DIR is
irrespective of the TEMP_CONFIG_DIR.
Merge pull request #129 from geoffgarside/freebsd-stable-etc-dir

Set the FreeBSD /etc/salt dir to PREFIX/etc/salt