Auto-generate PLATFORM_VERSION_ALL_CODENAMES.

Test: Temporarily added additional codenames for OPD1 -> O and
      OPM1 -> O-MR1. Lunched aosp_sailfish-eng, built build.prop and
      checked that ro.build.version.all_codenames was "O". Then
      lunched aosp_sailfish-eng-PPR1 and checked that it was
      "O,O-MR1,P".
Bug: None

Change-Id: I51189f5b40e3e7f99e2fc08bb5616c51a23e3b19