Currently there are two config options for building a U-boot binary for MX31PDK:
make mx31pdk_config
or,
make mx31pdk_nand_config
mx31pdk_config was developed first when no NAND SPL support was available for
MX31 and it requires that the U-boot binary is loaded into RAM via JTAG and it
forces SKIP_LOWLEVEL_INIT.
mx31pdk_nand_config was added later and it allows booting from NAND Flash.
Leave just one config option called mx31pdk so that it produces a binary that can boot from NAND Flash.