Platform/ARM/Sgi: fix incorrect check of config-id value

On SGI platform, the value of configuration ID can be zero.
So avoid returning an error from the function that creates
the system ID HOB in case the value of the configuration ID
is zero.

While at it, improve some of the error messages as well.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>