build: build Kconfig and config rules

build: build Kconfig and config rules

Wire in the Kconfig build and makefile rules to be able to generate
valid configuration files to be used by the build process but don't
actually use the output for affecting the Xen build. To avoid dragging
in most of Kbuild from the Linux kernel this adds Makefile.kconfig which
is our real entry point into building kconfig. This attempts to reuse as
much of the Xen build bits as possible and wire them to the bits that
kconfig expects to be provided by Kbuild.

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