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>

.gitignore

@@ -217,6 +217,8 @@ tools/xentrace/tbctl
 tools/xentrace/xenctx
 tools/xentrace/xentrace
 xen/.banner
+xen/.config
+xen/.config.old
 xen/System.map
 xen/arch/arm/asm-offsets.s
 xen/arch/arm/xen.lds
@@ -239,10 +241,14 @@ xen/include/headers++.chk
 xen/include/asm
 xen/include/asm-*/asm-offsets.h
 xen/include/compat/*
+xen/include/config/
+xen/include/generated/
 xen/include/public/public
 xen/include/xen/*.new
 xen/include/xen/acm_policy.h
 xen/include/xen/compile.h
+xen/tools/kconfig/.tmp_gtkcheck
+xen/tools/kconfig/.tmp_qtcheck
 xen/tools/symbols
 xen/xsm/flask/include/av_perm_to_string.h
 xen/xsm/flask/include/av_permissions.h