Add a new "none" platform that only builds utilities

Add a new "none" platform that only builds utilities

This makes it possible to build generally-useful utilities such as
grub-mount even if the rest of GRUB has not been ported to the target
CPU.

* configure.ac: Add "none" platform.  Default to it for unsupported
CPUs rather than stopping with a fatal error.  Don't downgrade
x86_64-none to i386.  Define COND_real_platform Automake conditional
if the platform is anything other than "none".  Don't do any include
directory linking for "none".
* Makefile.am: Skip building grub-core and all bootcheck targets if
!COND_real_platform.
* include/grub/time.h: Don't include <grub/cpu/time.h> if GRUB_UTIL
is defined.