env_sf: Move conditional compilation to Makefile

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/Makefile b/common/Makefile
index a9d66ae..e171ae0 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -56,7 +56,7 @@
 COBJS-$(CONFIG_ENV_IS_IN_NAND) += env_nand.o
 COBJS-$(CONFIG_ENV_IS_IN_NVRAM) += env_nvram.o
 COBJS-$(CONFIG_ENV_IS_IN_ONENAND) += env_onenand.o
-COBJS-y += env_sf.o
+COBJS-$(CONFIG_ENV_IS_IN_SPI_FLASH) += env_sf.o
 COBJS-y += env_nowhere.o
 
 # command