RECIPE FORMAT:
{CORENAME} {COREDIR} {URL} {REPOTYPE} {ENABLED} {COMMAND} {MAKEFILE} {SUBDIR} {ARGS} TRAILINGSPACE

Ex:
2048 libretro-2048 https://github.com/libretro/libretro-2048.git PROJECT YES GENERIC Makefile.libretro . 

The trailing space is really important for now, if no args are specified there needs to be a trailing space after SUBDIR
Command should usually be GENERIC and REPOTYPE should usually be project. Notable exceptions are PPSSPP and PICODRIVE. Those are submodule repos.