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>