build: fix clean targets when subdir-y is used

build: fix clean targets when subdir-y is used

The make variable $(subdir-y) isn't used yet but will be in a
following patch. Anything in $(subdir-y) doesn't to have a '/' as
suffix as we already now it's a directory.

Rework the rules so that it doesn't matter whether there is a '/' or
not. It also mimic more closely to the way Linux's Kbuild descend in
subdirectories.

FORCE phony target isn't needed anymore running clean, so it can be
removed.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>