Add empty C linux-style entry points

* On PPC, there are two different kernel entry signatures for Linux

src/system/boot/platform/u-boot/start.cpp

@@ -132,6 +132,23 @@ start_linux(int argc, int archnum, void *atags)
 }
 
 
+extern "C" int
+start_linux_ppc_old(void */*board_info*/,
+	void */*initrd_start*/, void */*initrd_end*/,
+	const char */*cmdline_start*/, const char */*cmdline_end*/)
+{
+	return 1;
+}
+
+
+extern "C" int
+start_linux_ppc_fdt(void *fdt, long/*UNUSED*/, long/*UNUSED*/,
+	uint32 epapr_magic, uint32 initial_mem_size)
+{
+	return 1;
+}
+
+
 extern "C" int
 start_raw(int argc, const char **argv)
 {