Allow packing custom radio images into target_files zip.

Radio images are added to INSTALLED_RADIOIMAGE_TARGET, which by default
will be packed under RADIO/ in a target_files zip. This CL introduces
BOARD_PACK_RADIOIMAGES that allows additionally copying them into
IMAGES/, which will then be included into <product>-img.zip.

Bug: 62195105
Test: Define BOARD_PACK_RADIOIMAGES and `m dist`. Check the generated
      target_files zip and <product>-img.zip.
Change-Id: I3deafd2dfecd1d4dbfdfc2d002fc40ef22fb60ea