build: prepare to always invoke $(MAKE) from xen/, use $(obj)

build: prepare to always invoke $(MAKE) from xen/, use $(obj)

In a future patch, when building a subdirectory, we will set
"obj=$subdir" rather than change directory.

Before that, we add "$(obj)" and "$(src)" in as many places as
possible where we will need to know which subdirectory is been built.
"$(obj)" is for files been generated during the build, and "$(src)" is
for files present in the source tree.

For now, we set both to "." in Rules.mk and Makefile.clean.

A few places don't tolerate the addition of "./", this is because make
remove the leading "./" in targets and dependencies in rules, so these
will be change later.

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