cmd_vfd: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/Makefile b/common/Makefile
index 8d4a426..3547766 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -144,7 +144,7 @@
 endif
 COBJS-$(CONFIG_CMD_XIMG) += cmd_ximg.o
 COBJS-$(CONFIG_YAFFS2) += cmd_yaffs2.o
-COBJS-y += cmd_vfd.o
+COBJS-$(CONFIG_VFD) += cmd_vfd.o
 COBJS-$(CONFIG_CMD_DOC) += docecc.o
 COBJS-y += flash.o
 COBJS-y += kgdb.o