[sbi] Add support for running as a RISC-V SBI payload

Add basic support for running directly on top of SBI, with no UEFI
firmware present.  Build as e.g.:

  make CROSS=riscv64-linux-gnu- bin-riscv64/ipxe.sbi

The resulting binary can be tested in QEMU using e.g.:

  qemu-system-riscv64 -M virt -cpu max -serial stdio \
                      -kernel bin-riscv64/ipxe.sbi

No drivers or executable binary formats are supported yet, but the
unit test suite may be run successfully.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/Makefile

@@ -11,6 +11,7 @@ INCDIRS		:= arch/$(ARCH)/include arch/riscv/include $(INCDIRS)
 #
 SRCDIRS		+= arch/riscv/core
 SRCDIRS		+= arch/riscv/interface/sbi
+SRCDIRS		+= arch/riscv/prefix
 
 # RISCV-specific flags
 #