fs: fat: Build only if CONFIG_FS_FAT=y

Signed-off-by: Ismael Luceno <ismael.luceno@silicon-gears.com>
diff --git a/fs/Makefile b/fs/Makefile
index 10c735a..2ed4aea 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -15,7 +15,7 @@
 obj-$(CONFIG_FS_CBFS) += cbfs/
 obj-$(CONFIG_CMD_CRAMFS) += cramfs/
 obj-$(CONFIG_FS_EXT4) += ext4/
-obj-y += fat/
+obj-$(CONFIG_FS_FAT) += fat/
 obj-$(CONFIG_FS_JFFS2) += jffs2/
 obj-$(CONFIG_CMD_REISER) += reiserfs/
 obj-$(CONFIG_SANDBOX) += sandbox/