platform/x86/intel/ifs: Refactor image loading code

IFS image loading flow is slightly different for newer IFS generations.

In preparation for adding support for newer IFS generations, refactor
portions of existing image loading code for reuse.

Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Link: https://lore.kernel.org/r/20231005195137.3117166-3-jithu.joseph@intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>