[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/config/console.h

@@ -41,6 +41,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 //#define	CONSOLE_VMWARE		/* VMware logfile console */
 //#define	CONSOLE_DEBUGCON	/* Bochs/QEMU/KVM debug port console */
 //#define	CONSOLE_INT13		/* INT13 disk log console */
+//#define	CONSOLE_SBI		/* RISC-V SBI debug console */
 
 /*
  * Very obscure console types