barebox: Allow to build an environment image

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
cfa-10036: Add a custom barebox environment

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
cfa10036: Update the flashing script to match the new partition layout

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>