Makefile: remove mx31pdk exception

An entry in the main Makefile for the mx31pdk board is not
necessary, and the board is added to boards.cfg as all
other targets.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>
diff --git a/boards.cfg b/boards.cfg
index a3b4c56..65482ac 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -41,6 +41,8 @@
 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
 mx35pdk                      arm         arm1136     -                   freescale      mx35
 omap2420h4                   arm         arm1136     -                   ti             omap24xx
 tnetv107x_evm                arm         arm1176     tnetv107xevm        ti             tnetv107x