spapr: Do not put empty properties for -kernel/-initrd/-append

We are going to use spapr_build_fdt() for the boot time FDT and as an
update for SLOF during handling of H_CAS. SLOF will apply all properties
from the QEMU's FDT which is usually ok unless there are properties
changed by grub or guest kernel. The properties are:
bootargs, linux,initrd-start, linux,initrd-end, linux,stdout-path,
linux,rtas-base, linux,rtas-entry. Resetting those during CAS will most
likely cause grub failure.

Don't create such properties if we're booting without "-kernel" and
"-initrd" so they won't get included into the DT update blob and
therefore the guest is more likely to boot successfully.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[dwg: Tweaked commit message based on Greg Kurz's input]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -1179,11 +1179,16 @@ static void spapr_dt_chosen(SpaprMachineState *spapr, void *fdt)
 
     _FDT(chosen = fdt_add_subnode(fdt, 0, "chosen"));
 
-    _FDT(fdt_setprop_string(fdt, chosen, "bootargs", machine->kernel_cmdline));
-    _FDT(fdt_setprop_cell(fdt, chosen, "linux,initrd-start",
-                          spapr->initrd_base));
-    _FDT(fdt_setprop_cell(fdt, chosen, "linux,initrd-end",
-                          spapr->initrd_base + spapr->initrd_size));
+    if (machine->kernel_cmdline && machine->kernel_cmdline[0]) {
+        _FDT(fdt_setprop_string(fdt, chosen, "bootargs",
+                                machine->kernel_cmdline));
+    }
+    if (spapr->initrd_size) {
+        _FDT(fdt_setprop_cell(fdt, chosen, "linux,initrd-start",
+                              spapr->initrd_base));
+        _FDT(fdt_setprop_cell(fdt, chosen, "linux,initrd-end",
+                              spapr->initrd_base + spapr->initrd_size));
+    }
 
     if (spapr->kernel_size) {
         uint64_t kprop[2] = { cpu_to_be64(KERNEL_LOAD_ADDR),