build: always use BASEDIR for xen sub-directory

build: always use BASEDIR for xen sub-directory

Modify Makefiles using $(XEN_ROOT)/xen to use $(BASEDIR) instead.

This is removing the dependency to xen subdirectory preventing using a
wrong configuration file when xen subdirectory is duplicated for
compilation tests.

BASEDIR is set in xen/lib/x86/Makefile as this Makefile is directly
called from the tools build and install process and BASEDIR is not set
there.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>