Use lzma compression for rootfs

This saves almost 30 percent.
diff --git a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
index 4a800dd..d7a681c 100644
--- a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
+++ b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
@@ -14,5 +14,5 @@
 MACHINEOVERRIDES =. "openbmc:"
 
 IMAGE_CLASSES += "obmc-phosphor-image_types_uboot"
-IMAGE_CTYPE = "gz"
+IMAGE_CTYPE = "lzma"
 IMAGE_FSTYPES += "cpio.${IMAGE_CTYPE}.u-boot"