Add as a separate file so that we can do "make bin/xxxx.kpxe" rather than

having to edit PXELOADER_KEEP_ALL as a config option.

src/arch/i386/prefix/kpxeprefix.S

+/*****************************************************************************
+ * PXE prefix that keep the PXE stack present
+ *****************************************************************************
+ */
+
+#define PXELOADER_KEEP_ALL
+#include "pxeprefix.S"