pmbootstrap install: support _pmb_groups

feedbackd uses in the latest version a separate group for controlling the LED indicator instead of the video group. The feedbackd package has been updated to create this group, but the user isn't added to it. See pmaports!1750 (merged)

This MR adds support for _pmb_groups in UI and UI-extra metapackages. If the additional groups are specified in these metapackages, the user will be added to these groups when using pmbootstrap install.