drivers/mtd/nand: Move conditional compilation to Makefile

rename CFG_NAND_LEGACY to CONFIG_NAND_LEGACY

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/ppc4xx/ndfc.c b/cpu/ppc4xx/ndfc.c
index 4f083d9..72acfd0 100644
--- a/cpu/ppc4xx/ndfc.c
+++ b/cpu/ppc4xx/ndfc.c
@@ -31,7 +31,7 @@
 
 #include <common.h>
 
-#if defined(CONFIG_CMD_NAND) && !defined(CFG_NAND_LEGACY) && \
+#if defined(CONFIG_CMD_NAND) && !defined(CONFIG_NAND_LEGACY) && \
 	(defined(CONFIG_440EP) || defined(CONFIG_440GR) ||	     \
 	 defined(CONFIG_440EPX) || defined(CONFIG_440GRX) ||	     \
 	 defined(CONFIG_405EZ) || defined(CONFIG_405EX) ||	     \