This start explainning the variables that can be used in the many
Makefiles in xen/.
Most of the document copies and modifies text from Linux v5.4 document
linux.git/Documentation/kbuild/makefiles.rst. Modification are mostly
to avoid mentioning kbuild. Thus I've added the SPDX tag which was
only in index.rst in linux.git.
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>