Game controller default mappings derived from https://github.com/mdqinc/SDL_GameControllerDB.
Non-Linux platform entries are removed and some MiSTer specific entries are added.
- platform 'MiSTer' added. In the event of conflict the MiSTer platform takes priority over 'Linux'.
- buttons 'menuok' and 'menuesc' are mappable. If these entries do not exist in the mapping 'a' and 'b' buttons are mapped to these functions.
- Core based matching. 'mistercore:' limits the mapping entry to only the named core. Wildcards are supported. A database linecan have multiple mistercore entries.