board: freescale: various boards: Let env subsystem set gd->env_addr

Various freescale boards set gd->env_addr to default_environment in
board_init(), conditional on CONFIG_ENV_IS_NOWHERE, but this is
redundant, since it is done by env_init() before board_init() is called.

Let the env subsystem handle this.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Mian Yousaf Kaukab <ykaukab@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>