env_flash: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/Makefile b/common/Makefile
index fad9bc1..a6fd97e 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -52,7 +52,7 @@
 COBJS-$(CONFIG_ENV_IS_IN_DATAFLASH) += env_dataflash.o
 COBJS-$(CONFIG_ENV_IS_IN_EEPROM) += env_eeprom.o
 COBJS-y += env_embedded.o
-COBJS-y += env_flash.o
+COBJS-$(CONFIG_ENV_IS_IN_FLASH) += env_flash.o
 COBJS-$(CONFIG_ENV_IS_IN_NAND) += env_nand.o
 COBJS-$(CONFIG_ENV_IS_IN_NVRAM) += env_nvram.o
 COBJS-$(CONFIG_ENV_IS_IN_ONENAND) += env_onenand.o