opos6uldev: Migrate to using CONFIG_EXTRA_ENV_TEXT

Move the environment text over from being set via
CONFIG_EXTRA_ENV_SETTINGS in include/configs/opos6uldev.h and over to
plain text in board/armadeus/opos6uldev/opos6uldev.env.  This lets us
manage env_version without a CONFIG variable.

Cc: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Tom Rini <trini@konsulko.com>