build: hook kconfig into xen build system

build: hook kconfig into xen build system

Now that xen's build system is very close to Linux's ones, we can hook
"Makefile.host" into Xen's build system, and we can build Kconfig with
that.

"tools/kconfig/Makefile" now needs a workaround to not rebuild
"$(XEN_ROOT)/.config", as `make` tries the rules "%.config" which
fails with:
    tools/kconfig/Makefile:95: *** No configuration exists for this target on this architecture.  Stop.

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