cmd: Add Kconfig option for CMD_MTDPARTS and related options

CMD_MTDPARTS is something the user might or might not want to select, and
might depends on (or be selected by) other options too.

This is even truer for the MTDIDS_DEFAULT and MTDPARTS_DEFAULT options that
might change from one board to another, or from one user to the other,
depending on what it expects and what storage devices are available.

In order to ease that configuration, add those options to Kconfig.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>