mx31pdk: Remove unneeded config

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.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>

boards.cfg

@@ -41,8 +41,7 @@ mx31ads                      arm         arm1136     -                   freesca
 imx31_litekit                arm         arm1136     -                   logicpd        mx31
 imx31_phycore                arm         arm1136     -                   -              mx31
 imx31_phycore_eet            arm         arm1136     imx31_phycore       -              mx31         imx31_phycore:IMX31_PHYCORE_EET
-mx31pdk_nand                 arm         arm1136     mx31pdk             freescale      mx31         mx31pdk:NAND_U_BOOT
-mx31pdk                      arm         arm1136     -                   freescale      mx31         mx31pdk:SKIP_LOWLEVEL_INIT
+mx31pdk                      arm         arm1136     -                   freescale      mx31         mx31pdk:NAND_U_BOOT
 mx35pdk                      arm         arm1136     -                   freescale      mx35
 omap2420h4                   arm         arm1136     -                   ti             omap24xx
 tnetv107x_evm                arm         arm1176     tnetv107xevm        ti             tnetv107x