cmd: mtdparts: use defaults by default

Boards which are defining default mtdparts often need them early
in boot process (to load environment from UBI volume, for example).
This is currently solved by adding mtdparts and mtdids variable
definitions also to default environment. With this change, default
partitions are used by default unless explicitely deleted or
redefined.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>