[CHANGELOG] Improve search for MRs to be collected

* Since now, for each project, the script was searching only for MRs
that were merged into branches having the same name of the manifest main
branch.
* In order to correct this approach, we define a function that searches
  for the INTEGRATION variable in each project and get the name of
the branch whose changes are integrated into the manifest main branch. If
more than one branch integrates into the manifest one, all of them are
considered in the search for MRs to be inserted in the changelog.
* Include BSP repositories and new meta-layers in CHANGELOG_PROJECTS
  variable