boards: add a 'none' machine type to all platforms

This allows any QEMU binary to be executed with:

  $QEMU_BINARY -M none -qmp stdio

Without errors from missing options that are required by various boards.  This
also provides a mode that we can use in the future to construct machines
entirely through QMP commands.

Cc: Daniel Berrange <berrange@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/Makefile.objs

@@ -134,6 +134,8 @@ hw-obj-$(CONFIG_DP8393X) += dp8393x.o
 hw-obj-$(CONFIG_DS1225Y) += ds1225y.o
 hw-obj-$(CONFIG_MIPSNET) += mipsnet.o
 
+hw-obj-y += null-machine.o
+
 # Sound
 sound-obj-y =
 sound-obj-$(CONFIG_SB16) += sb16.o

hw/null-machine.c

@@ -0,0 +1,40 @@
+/*
+ * Empty machine
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Anthony Liguori   <aliguori@us.ibm.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu-common.h"
+#include "hw/hw.h"
+#include "hw/boards.h"
+
+static void machine_none_init(ram_addr_t ram_size,
+                              const char *boot_device,
+                              const char *kernel_filename,
+                              const char *kernel_cmdline,
+                              const char *initrd_filename,
+                              const char *cpu_model)
+{
+}
+
+static QEMUMachine machine_none = {
+    .name = "none",
+    .desc = "empty machine",
+    .init = machine_none_init,
+    .max_cpus = 0,
+};
+
+static void register_machines(void)
+{
+    qemu_register_machine(&machine_none);
+}
+
+machine_init(register_machines);
+