flatpak-builder: Make the runtime extension commit ids part of the manifest checksum

[alexlarsson]

From @alexlarsson on May 25, 2016 15:4

This will force us to rebuild if some extension runtime that will be used was updated on installed.

Copied from original issue: flatpak/flatpak#38

[alexlarsson]

Note: This should only happen if we're building a runtime or something and we're actually copying the extensions into the result, or if you pass --rebuild-on-sdk-change

[nanonyme]

@alexlarsson what about apps with Sdk extensions?