Add RISC-V Support

This is initial patches to add RISC-V support to AGL.

This should allow building AGL for the qemuriscv64 MACHINE.

Tested with the latest branches setup by running repo following
the AGL instructions.

This was also tested with the latest master OE.

Currently I can boot console only images, the GUI isn't working yet,
although the splash screen works.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Change-Id: I89ef13da8fb273f2a016fa3d1f0735f4c2a54bec
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25106
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>