hw/i386/pc: Rename pc_build_feature_control() as generic fw_cfg_build_*

Now that the pc_build_feature_control_file() function has been
refactored to not depend of PC specific types, rename it to a
more generic name.

Suggested-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-15-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>