Fix ordering of selected versions when assembling the overlay

[ziopio]

The function collect_overlay was ordering the directory list but it was also missing 2 things:

• ordering directories was putting the versions in order but that pushed "common" at the end of the list
  This was hiding the next bug when just one version folder is selected:
• select_overlay_folders was not reversing the order of the list after the tail recursion

Now instead we do:

• list everything,
• filter out everything that is not an otp version folder
• order such list
• select
• reverse selection order (to fix ordering)
• put "common" at the head of the list