ohos-xts-acts: Build jffs2 test image

jffs2.img will be used to mount /storage directory required by ActsJFFS2Test
test suite using loopback device.

Related-To: https://git.ostc-eu.org/OSTC/OHOS/components/staging/xts_acts/-/issues/5


Signed-off-by: Robert Drab <robert.drab@huawei.com>