futility: updater: refactor 'archive' structure name

Rename 'struct archive' to 'struct u_archive' as 'updater archive'
so in future we can support libarchive (which needs 'struct archive').

BUG=None
TEST=make; run test
BRANCH=None

Change-Id: Id4e5b425044b6f1e64f504373c264dba05da62f4
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3644539
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>