build: replace $(BASEDIR) and use $(srctree)

build: replace $(BASEDIR) and use $(srctree)

$(srctree) is a better description for the source directory than
$(BASEDIR) that has been used for both source and build directory
(which where the same).

This adds $(srctree) to a few path where make's VPATH=$(srctree) won't
apply. And replace $(BASEDIR) by $(srctree).

Introduce "$(srcdir)" as a shortcut for "$(srctree)/$(src)" as the
later is used often enough.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com> # XSM