The mptopdf script does standalone conversion from mpost to
PDF, using the supp-* and syst-* files. They also allow native
MetaPost graphics inclusion in LaTeX (via pdftex.def) and
ConTeXt. They can be used independently of the rest of ConTeXt,
yet are maintained as part of it. So in TeX Live we pull them
out to this separate package for the benefit of LaTeX users who
do not install the rest of ConTeXt. This can be found on CTAN
in macros/pdftex/graphics.