[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/config/defaults/sbi.h

@@ -0,0 +1,36 @@
+#ifndef CONFIG_DEFAULTS_SBI_H
+#define CONFIG_DEFAULTS_SBI_H
+
+/** @file
+ *
+ * Configuration defaults for RISC-V SBI
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#define IOAPI_RISCV
+#define IOMAP_VIRT
+#define DMAAPI_FLAT
+#define UACCESS_FLAT
+#define TIMER_ZICNTR
+#define ENTROPY_ZKR
+
+#define CONSOLE_SBI
+#define REBOOT_SBI
+#define UMALLOC_SBI
+
+#define ACPI_NULL
+#define MPAPI_NULL
+#define NAP_NULL
+#define PCIAPI_NULL
+#define SANBOOT_NULL
+#define SMBIOS_NULL
+#define TIME_NULL
+
+#define IMAGE_SCRIPT
+
+#define REBOOT_CMD
+#define POWEROFF_CMD
+
+#endif /* CONFIG_DEFAULTS_SBI_H */