This commit adds support for replacing any partition image within
the payload with a custom image. This is useful, for example, to add a
custom kernel to an OTA, which may involve partitions that wouldn't
normally be touched (eg. `vendor_dlkm`).

Any image specified via `--replace` will have its corresponding
descriptor in the vbmeta image updated. This is handled recursively. For
example, replacing `vendor_dlkm` would update both `vbmeta_vendor` and
`vbmeta`. This requires all vbmeta images to be extracted during the
patching process so that a complete dependency graph can be computed.
The performance hit in doing so is negligible, but does require the
checksums of the stripped images to be updated for the tests.

Fixes: #102

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>