mtd: Hide CONFIG_MTD_BLKDEVS from the menu

Make this option a hidden one and get a cleaner configuration.
This option just selects a common infrastructure for MTD-based devices
to expose a block interface. There is no point in allowing a separate
enable/disable.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
[Brian: keep symbol as tristate]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>