[arch-releng] Nicer boot menu entry on Macs

Hi guys, This might be an edge case, so feel free to reject the idea. Currently when entering the boot menu on a Mac to install Arch, the correct entry to select has a generic usb harddrive logo and the label "Windows". I don't find this hugely helpful. If we install the attached icon as '.VolumeIcon.icns' on the EFI System Partition on the install media we get a nice Arch logo rather than the generic one. Changing the label from "Windows" to "Arch Linux Install" (or something like that), is left for future work. I'll look into that if unless there are objections to the icon. Cheers, Tom