This builds the x64 and i386 EDK2 OVMF firmware for the QEMU emulator, installing
the files QEMU_UEFI_CODE-${arch}.fd and QEMU_UEFI_VARS-${arch}.fd.
These can be used with the pflash device to separate the firmware into
readonly firmware and read-write persistent variable storage areas.
Note: I named the files QEMU_UEFI_*.fd to follow the existing scheme used by bhyve. I'm not sure if we want to mention OVMF instead, since that's what we build?