swift: Generate a mmc ext4 tar image file

Swift has an emmc chip, add the directive to generate a tarball for it.

Change-Id: I9c4e2c78994ba5f125f202ae00775e6d0761146d
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/meta-witherspoon/conf/machine/swift.conf b/meta-witherspoon/conf/machine/swift.conf
index 3915e2b..4c0abb5 100644
--- a/meta-witherspoon/conf/machine/swift.conf
+++ b/meta-witherspoon/conf/machine/swift.conf
@@ -1,6 +1,6 @@
 require conf/machine/witherspoon.conf
 
-IMAGE_FSTYPES += "ext4"
+IMAGE_FSTYPES += "ext4 mmc-ext4-tar"
 
 KERNEL_DEVICETREE = "${KMACHINE}-bmc-opp-${MACHINE}.dtb"