Resolve "Add FreeIntv (libretro emulation core for the Mattel Intellivision)"

Please make sure your MR is ready to be merged !

• You added the changes in CHANGELOG.md
• You described the MR as below
• You updated the recalbox wiki
• You wrote a blog post on recalbox-blog

Changes :

• Adds libretro´s FreeIntv Mattel Intellivision support
• Theme exists already

Related to :

• recalbox-configgen!182 (merged)

Closes #406 (closed)