seal_oem_partition: make cmdline updates compatible with pickargs

Update behavior to not assume device mapper table is at the end of the
command line.

BUG=b/517096251

Change-Id: I9842c48689127dac7f8aad2a87a30354fd672250
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/156144
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: He Gao <hegao@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Miri Amarilio <mirilio@google.com>