Simply add the two lines to the ebuild in the install section. There is only
bmps in the boinc source, so I took the boinc128.bmp file and added alpha and
saved it as an xpm. You could convert the icon from the source but you would
still end up with an ugly white background (unless you know voodoo I don't ;).
It would be preferable to put the boinc.xpm into the SRC_URI instead of portage,
but I don't have anywhere to host it to get it off the ground...
doicon ${FILESDIR}/boinc.xpm
make_desktop_entry boinc_gui "BOINC Manager" boinc.xpm "Science;" /var/lib/boinc/