build: shuffle main Makefile

build: shuffle main Makefile

Reorganize a bit the Makefile ahead of patch
"build: adding out-of-tree support to the xen build"

We are going to want to calculate all the $(*srctree) and $(*objtree)
once, when we can calculate them. This can happen within the
"$(root-make-done)" guard, in an out-of-tree build scenario, so move
those variable there.

$(XEN_ROOT) is going to depends on the value of $(abs_srctree) so
needs to move as well. "Kbuild.include" also depends on $(srctree).

Next, "Config.mk" depends on $(XEN_ROOT) and $(TARGET_*ARCH) depends
on "Config.mk" so those needs to move as well.

This should only be code movement without functional changes.

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