Make IPFire 2 build on riscv64

Hello everyone,

this is as promised the patchset that makes IPFire build on (an emulated) riscv64 platform.

As stated in my previous email, this is not ready for primetime since we do not have any hardware, nor are we expecting any in the very near future.

For that reason, I was toying around with this and wanted to know whether this builds and get a feeling for this architecture.

Rootfiles for some packages - including the kernel - are missing, which is why the generated won't boot at all. This is not a problem since we don't have any hardware, but I wanted to note this.

Please merge this. There is very little chance this would break anything.

-Michael