config: Add default client arch defines for intel architectures

Define default PXE client architecture identifiers for IA32 (0x0 aka
Intel x86PC) and Intel x86-64 (0x9 aka EFI x86-64).

This prepares for usage for config_distro_defaults in the sandbox
architecture

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>