cmd_nand: Move conditional compilation to Makefile

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

common/Makefile

diff --git a/common/Makefile b/common/Makefile
index 3781738..b8c596c 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -116,7 +116,7 @@
 COBJS-$(CONFIG_CMD_MMC) += cmd_mmc.o
 COBJS-$(CONFIG_MP) += cmd_mp.o
 COBJS-$(CONFIG_CMD_MTDPARTS) += cmd_mtdparts.o
-COBJS-y += cmd_nand.o
+COBJS-$(CONFIG_CMD_NAND) += cmd_nand.o
 COBJS-$(CONFIG_CMD_NET) += cmd_net.o
 COBJS-$(CONFIG_CMD_ONENAND) += cmd_onenand.o
 COBJS-$(CONFIG_CMD_OTP) += cmd_otp.o