Re-factoring the legacy NAND code (legacy NAND now only in board-specific

code and in SoC code). Boards using the old way have CFG_NAND_LEGACY and
BOARDLIBS = drivers/nand_legacy/libnand_legacy.a added. Build breakage for
NETTA.ERR and NETTA_ISDN - will go away when the new NAND support is
implemented for these boards.