Note: There is a bug in EmuELEC 3.8 that makes DOOM 1 use the wrong location. On EmuELEC 3.8 you should place doom1.wad as /storage/roms/ports/doom2/doom1.wad instead. This is fixed since https://github.com/EmuELEC/EmuELEC/pull/308.

Official DOOM

You will need to get the .wad files for the DOOM version you want to play and place them in the following locations:

DOOM: /storage/roms/ports/doom/doom1.wad
DOOM II: /storage/roms/ports/doom2/doom2.wad

You will have to extract these files from a legitimately bought copy of DOOM or DOOM II. See sites like https://doomwiki.org/wiki/Steam#How_to_use_source_ports_with_games_downloaded_from_Steam for more info.

FreeDOOM

FreeDOOM is a 100% Open Source asset pack for DOOM and DOOM II, with its own set of levels. To play FreeDOOM instead of official DOOM, go to https://freedoom.github.io/download.html and download the .zip.

For DOOM, rename freedoom1.wad to doom1.wad and place it in /storage/roms/ports/doom/.
For DOOM II, rename freedoom2.wad to doom2.wad and place it in /storage/roms/ports/doom2/.