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/doc/README.nand b/doc/README.nand
index 0ad5e18..171380e 100644
--- a/doc/README.nand
+++ b/doc/README.nand
@@ -184,7 +184,7 @@
 The old NAND handling code has been re-factored and is now confined
 to only board-specific files and - unfortunately - to the DoC code
 (see below). A new configuration variable has been introduced:
-CFG_NAND_LEGACY, which has to be defined in the board config file if
+CONFIG_NAND_LEGACY, which has to be defined in the board config file if
 that board uses legacy code.
 
 The necessary changes have been made to all affected boards, and no