build: adding out-of-tree support to the xen build

This implement out-of-tree support, there's two ways to create an
out-of-tree build tree (after that, `make` in that new directory
works):
    make O=build
    mkdir build; cd build; make -f ../Makefile
also works with an absolute path for both.

This implementation only works if the source tree is clean, as we use
VPATH.

This patch copies most new code with handling out-of-tree build from
Linux v5.12.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Tested-by: Julien Grall <jgrall@amazon.com>
Acked-by: Ross Lagerwall <ross.lagerwall@citrix.com> # livepatch

.gitignore

@@ -325,6 +325,7 @@ xen/include/xen/*.new
 xen/include/xen/acm_policy.h
 xen/include/xen/compile.h
 xen/include/xen/lib/x86/cpuid-autogen.h
+xen/source
 xen/test/livepatch/config.h
 xen/test/livepatch/expect_config.h
 xen/test/livepatch/*.livepatch