The install and upgrade scripts are both symbolic links to install.sub, the source to that script is in src/distrib/miniroot/install.sub. There are also architecture stubs with machine dependent bits located in src/distrib/<arch>/<varies>/install.md.
Here's a link to the -current install.sub at this time, revision 1.1060:
http://cvsweb.openbsd.org/cgi-bin/cv...ype=text/plain