mtd: nand: dynamic allocation of flash-based BBT structs

It is nicer to dynamically create our badblock patterns than to
statically define them. The nand_create_default_bbt_descr() function
does a sufficient job of handling various bad block scanning options
for either flash-based or non-flash-based BBTs, so we might as well
use the function for both cases.

This patch simplifies and shortens our code (and removes a TODO that
I left a few months ago).

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>