eclass: set default portage user and group

The PORTAGE_USERNAME and PORTAGE_GRPNAME variables are not exported by
default. The SDK uses them to get portage to run builds as the
developer's uid/gid instead of portage:portage. When running outside of
the SDK use the default portage user and group.
Merge pull request #757 from marineam/user

eclass: set default portage user and group