cmd_mem: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/Makefile b/common/Makefile
index a6fd97e..8bddf8e 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -113,7 +113,7 @@
 COBJS-y += cmd_load.o
 COBJS-$(CONFIG_LOGBUFFER) += cmd_log.o
 COBJS-$(CONFIG_ID_EEPROM) += cmd_mac.o
-COBJS-y += cmd_mem.o
+COBJS-$(CONFIG_CMD_MEMORY) += cmd_mem.o
 COBJS-$(CONFIG_CMD_MFSL) += cmd_mfsl.o
 COBJS-$(CONFIG_MII) += miiphyutil.o
 COBJS-$(CONFIG_CMD_MII) += miiphyutil.o