[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/riscv64/Makefile.sbi

@@ -0,0 +1,6 @@
+# -*- makefile -*- : Force emacs to use Makefile mode
+
+# Include generic SBI Makefile
+#
+MAKEDEPS	+= arch/riscv/Makefile.sbi
+include arch/riscv/Makefile.sbi

src/arch/riscv64/include/ipxe/sbi/dhcparch.h

@@ -0,0 +1,20 @@
+#ifndef _IPXE_SBI_DHCPARCH_H
+#define _IPXE_SBI_DHCPARCH_H
+
+/** @file
+ *
+ * DHCP client architecture definitions
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/dhcp.h>
+
+/** DHCP client architecture */
+#define DHCP_ARCH_CLIENT_ARCHITECTURE DHCP_CLIENT_ARCHITECTURE_RISCV64
+
+/** DHCP client network device interface */
+#define DHCP_ARCH_CLIENT_NDI 1 /* UNDI */ , 3, 10 /* v3.10 */
+
+#endif /* _IPXE_SBI_DHCPARCH_H */