apt-get dist-upgrade brakes down because

1) there is a libffi5 upgrade in maemo extras
2) there is a explicit depency on specific libffi5 version in the "tag-all" metapackage (mp-fremantle-generic-pr)
3) apt-get dist-upgrade decides to pull out mp-fremantle-generic-pr in favour or installing new libffi5
4) apt-get notices that a big bunch of packages can now be removed as only mp-fremantle-generic-pr depended on them

I think apt is being buggy at step #4, there should be no reason to remove those packages. Still, some protections should be set:

1) mark essential packages essential (duh) and apt won't remove them
2) maemo extras should refuse packages that are already included in the image...