Convert CONFIG_SYS_NAND_BAD_BLOCK_POS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_NAND_BAD_BLOCK_POS

In order to do this, introduce a choice for HAS_LARGE/SMALL_BADBLOCK_POS
as those are the only valid values.  Use LARGE as the default as no
in-tree boards use SMALL, but it is possible.

Signed-off-by: Tom Rini <trini@konsulko.com>