imagebuilder: add function to show manifest

Tested with 18.06.0-rc2/ar71xx/generic/tl-wdr4300-v1, image & list

This PR is based on the work of @fewckert[1] with slight improvements.

Add function `manifest` to show the manifest of the produced image,
before actually building it. The manifest contains an orderd list of
package name and version.

This is usefull to check package dependencies but also determine a
unique and reproducible image name before building the package. The
sysupgrade server[2] builds images on request with individual package
selection. To distignish between created images which contain differnt
packages, the EXTRA_IMAGE_NAME is set to a shortend hash of the
manifest's content. So far the image was renamed afterwards as the
manifests content was unknown, however this corrupts the signed
sha256sums. This patch allows a clean solution as to dtermine the
manifest in advance and set the EXTRA_IMAGE_NAME accordingly.

[1]: https://github.com/lede-project/source/pull/1591
[2]: https://github.com/aparcar/attendedsysupgrade-server

Signed-off-by: Paul Spooren <mail@aparcar.org>

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