xen/build: Use a distro version of figlet

xen/build: Use a distro version of figlet

It is quite inappropriate to keep a hacked up versions of figlet in our source
tree, especially when the purpose of the hackary is just to provide a text to
octal conversion.

This version of figlet contributes a surprisingly large proportion of the
Coverity issues found under xen/ (and therefore attributed against Xen)

Figlet can be found in all distros, so make use of it.  We keep xen.flf (being
the Xen figlet font) and replace the hacked up octal transform with a short
python script.

The Xen Makefile has been tweaked in such a way that it still prints the
figlet banner for the build.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>