xen/banner: Drop the fig-to-oct.py script

xen/banner: Drop the fig-to-oct.py script

The script is 664 rather than 775, so the banner conversion doesn't actually
work if $(PYTHON) is empty:

  /bin/sh: tools/fig-to-oct.py: Permission denied
  make[3]: *** [include/xen/compile.h] Error 126
  make[3]: Leaving directory `/builds/xen-project/people/andyhhp/xen/xen'

Fixing this is easy, but using python here is wasteful.  compile.h doesn't
need XEN_BANNER rendering in octal, and text is much more simple to handle.
Replace fig-to-oct.py with a smaller sed script.  This could be a shell
one-liner, but it is much more simple to comment sensibly, and doesn't need to
include the added cognative load of makefile and shell escaping.

While changing this logic, take the opportunity to optimise the banner
space (and time on the serial port) by dropping trailing whitespace, which is
84 characters for current staging.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>