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/arm920t/s3c24x0/nand.c b/cpu/arm920t/s3c24x0/nand.c
index e1bf128..14882cb 100644
--- a/cpu/arm920t/s3c24x0/nand.c
+++ b/cpu/arm920t/s3c24x0/nand.c
@@ -27,7 +27,7 @@
 #endif
 
 #if defined(CONFIG_CMD_NAND)
-#if !defined(CFG_NAND_LEGACY)
+#if !defined(CONFIG_NAND_LEGACY)
 
 #include <nand.h>
 #include <s3c2410.h>