at91: Choose environment variables location within make config target

This patch adds the possiblity to choose the media where the environment will
be located. This allow to choose this fundamental configuration without editing
config files.

Documentation file added.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Stelian Pop <stelian@popies.net>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>