Bug#560415: Merge

[Filipus Klutiero]

merge 553999 560415
thanks

Both of these issues are asking for the same workaround.

Instead of a blacklist, os-prober could maintain a list of other OS-es. 
Each item would have to be accepted or rejected. For example, during the 
installation, if the GRUB prompt is accepted, all detected operating 
systems would be accepted.
Then, when a new other operating system would be detected, os-prober 
could prompt whether to accept or reject it.