I personally like using a program called Goomba Color (just google is), which allows you convert GB or GBC roms into GBA roms. It also has an awesome feature that allows you to create a single
multi-rom.. Say for example you have 30 of your favorite GB roms, you can create 1 master GBA single rom. Once the rom is in GBA format, then it's easy to use something like PhacoxsInjector (my fav) or UWUVCI to create a GBA Wiiu VC.
On the WiiU side, the rom will only take up 1 title ID and upon launch it will give you a listing of the 30+ roms to select from..
WiiU VC Menu would look something like
this