mtd: nand: rename CREATE_EMPTY bbt flag with proper prefix

According to our new prefix rules, we should rename NAND_CREATE_EMPTY_BBT
with a NAND_BBT prefix, i.e., NAND_BBT_CREATE_EMPTY.

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