nand: mxc: Switch NAND SPL to generic SPL

This also fixes support for mx31pdk and tx25, which had been broken by commit
e05e5de7fae5bec79617e113916dac6631251156.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/board/freescale/mx31pdk/Makefile b/board/freescale/mx31pdk/Makefile
index 5b7cafd..b910722 100644
--- a/board/freescale/mx31pdk/Makefile
+++ b/board/freescale/mx31pdk/Makefile
@@ -27,6 +27,9 @@
 
 LIB	= $(obj)lib$(BOARD).o
 
+ifdef CONFIG_SPL_BUILD
+SOBJS	:= lowlevel_init.o
+endif
 COBJS	:= mx31pdk.o
 
 SRCS	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)