xen: rework `checkpolicy` detection when using "randconfig"

xen: rework `checkpolicy` detection when using "randconfig"

This patch allows to easily add more override which depends on the
environment.

Also, move the check out of Config.mk and into xen/ build system.
Nothing in tools/ is using that information as it's done by
./configure.

We named the new file ".allconfig.tmp" as ".*.tmp" are already ignored
via .gitignore.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>