env_onenand: set ONENAND_MAX_ENV_SIZE to CONFIG_ENV_SIZE

This fix prevents env_import() CRC to fail when CONFIG_ENV_SIZE
is not equal to 4096 bytes
It also prevents mtd->read and mtd->write to be incomplete when
the environment is larger than 4096 bytes.

Signed-off-by: David du Colombier <0intro@gmail.com>