build: use obj-y += subdir/ instead of subdir-y

build: use obj-y += subdir/ instead of subdir-y

This is part of upgrading our build system and import more of Linux's
one.

In Linux, subdir-y in Makefiles is only used to descend into
subdirectory when there are no object to build, Xen doesn't have that
and all subdir have object to be included in the final binary.

To allow the new syntax, the "obj-y" and "subdir-*" calculation in
Rules.mk is changed and partially imported from Linux's Kbuild.

The command used to modify the Makefile was:
    sed -i -r 's#^subdir-(.*)#obj-\1/#;' **/Makefile

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>