build: tweak variable exporting for make 3.82

build: tweak variable exporting for make 3.82

While I've been running into an issue here only because of an additional
local change I'm carrying, to be able to override just the compiler in
$(XEN_ROOT)/.config (rather than the whole tool chain), in
config/StdGNU.mk:

ifeq ($(filter-out default undefined,$(origin CC)),)

I'd like to propose to nevertheless correct the underlying issue:
Exporting an unset variable changes its origin from "undefined" to
"file". This comes into effect because of our adding of -rR to
MAKEFLAGS, which make 3.82 wrongly applies also upon re-invoking itself
after having updated auto.conf{,.cmd}.

Move the export statement past $(XEN_ROOT)/config/$(XEN_OS).mk inclusion
(which happens through $(XEN_ROOT)/Config.mk) such that the variables
already have their designated values at that point, while retaining
their initial origin up to the point they get defined.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>