env_dataflash: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/Makefile b/common/Makefile
index fea0caf..27f515b 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -50,7 +50,7 @@
 # environment
 COBJS-y += environment.o
 COBJS-y += env_common.o
-COBJS-y += env_dataflash.o
+COBJS-$(CONFIG_ENV_IS_IN_DATAFLASH) += env_dataflash.o
 COBJS-$(CONFIG_ENV_IS_IN_EEPROM) += env_eeprom.o
 COBJS-y += env_flash.o
 COBJS-y += env_nand.o