[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/config.c

@@ -78,6 +78,9 @@ REQUIRE_OBJECT ( vmconsole );
 #ifdef CONSOLE_DEBUGCON
 REQUIRE_OBJECT ( debugcon );
 #endif
+#ifdef CONSOLE_SBI
+REQUIRE_OBJECT ( sbi_console );
+#endif
 
 /*
  * Drag in all requested network protocols