winpe: make sure second stage boot manager actually exists

The same boot manager is used for both stages. A dfferent file name
is only used to detect the loading of the second stage to swap out
the served BCD.

This should have been part of 28bfedb610
where the instructions were changed from the manual to the GDB based
method.

pxe/start-pxe-winpe.sh

@@ -23,8 +23,8 @@ condition 2 $_regex(file, ".*/bootmgfw-stage2\\.efi$")
 commands 2
   print "Preparing stage 2"
   disable 2
-  # would restore initial state, but might interfere, restart script instead
   enable 1
+  shell ln -sf bootmgfw.efi tftp/bootmgfw-stage2.efi
   shell ln -sf BCD_winpe2 smb/BCD
   continue
 end