[riscv] Add support for the SBI debug console

Add the ability to issue Supervisor Binary Interface (SBI) calls via
the ECALL instruction, and use the SBI DBCN extension to implement a
debug console.

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

src/arch/riscv/Makefile

@@ -10,6 +10,7 @@ INCDIRS		:= arch/$(ARCH)/include arch/riscv/include $(INCDIRS)
 # RISCV-specific directories containing source files
 #
 SRCDIRS		+= arch/riscv/core
+SRCDIRS		+= arch/riscv/interface/sbi
 
 # RISCV-specific flags
 #