Kconfig: fix environment variable handling

Kconfig: fix environment variable handling

With xen/Makefile including include/config/auto.conf.cmd, environment
variables checked in the latter must be available at the time of
inclusion of that file, and hence must be populated in xen/Makefile
rather than by passing to or inside xen/tools/kconfig/Makefile.kconfig.
Otherwise incremental re-builds will always be full re-builds, which is
not only annoying but actively problematic when building as non-root
and only running "install-xen" as root.

Also take the opportunity and remove stray $(Q) uses.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>