setup-environment: Stop displaying imx6qsabresd when already configured

When a build dir is already configured, using
'. ./setup-environment build' will display:
"Configuring for imx6qsabresd"
and
"Your configuration files at build have not been touched."
which is contradictory.

So test for an existing configuration before displaying the machine

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>