distro: Extend with RISC-V defines

While we don't have UEFI naming conventions for RISC-V file paths yet,
we need to search for something. So let's copy the removable file paths
from the RISC-V edk2 port.

Also add the official VCI strings that contain the standardized RISC-V
architecture ID fields.

Signed-off-by: Alexander Graf <agraf@suse.de>