ifplop.c32: Detect if PLoP USB/CD INT13h hook is enabled/disabled

ifplop.c32: Detect if PLoP USB/CD INT13h hook is enabled/disabled

New module which detects if the PLoP Boot Loader already has booted a
CDROM or USB drive by checking for the presence of the PLoP INT13h hook.

The following assembly code (NASM) can detect the PLoP INT13h hook:

  mov eax,'PoLP'  ; Reverse of 'PLoP'
  mov ebp,'DKHC'  ; Reverse of 'CHKD'
  int 13h
  cmp eax,' sey'  ; Reverse of 'yes '
  jz plop_INT13h_active

Signed-off-by: Gert Hulselmans <gerth@zytor.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>