[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/riscv/Makefile.sbi

@@ -0,0 +1,16 @@
+# -*- makefile -*- : Force emacs to use Makefile mode
+
+# Build a position-independent executable, with relocations required
+# only for data values.  Runtime relocations are applied by the
+# prefix code.
+#
+CFLAGS		+= -mcmodel=medany -fpie
+LDFLAGS		+= -pie --no-dynamic-linker
+
+# Linker script
+#
+LDSCRIPT	= arch/riscv/scripts/sbi.lds
+
+# Media types
+#
+MEDIA		+= sbi