Make tarball path overridable

- Set TARBALL_FETCH_ROOT_DIRECTORY with ?= so we can override
- Move the /rootfs path path into TARBALL_FETCH_ROOT_DIRECTORY
- Set revision variables to ., as Kokoro just drops all the x20
resources directly into KOKORO_GFILE_DIR

Change-Id: Ic2c3a18b3f72471efddee4920ced673101d9a703
4 files changed