Make build meta-information available in build

[matthiasbeyer]

This might hurt reusability if meta-information is interpolated into the packaging script (e.g. date), but maybe it is worth considering making such metadata available.

For example:

• Date
• "root" package of the build

more?

[christophprokop]

related to #337 but this issue does not provide more info/requests

[primeos-work]

I don't think this is related to the other issue. #337 is about supporting metapackages (a special kind of package) while this issue seems to be about making meta-information available to packages (during the build).

I assume that we currently don't need this feature though (apart from maybe SOURCE_DATE_EPOCH for #349).