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

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>