build: rework "clean" to clean from the root dir

build: rework "clean" to clean from the root dir

This will allow "clean" to work from an out-of-tree build when
it will be available.

Some of the file been removed in current "clean" target aren't added
to $(clean-files) because they are already listed in $(extra-) or
$(extra-y).

Also start to clean files listed in $(targets). This allows to clean
"common/config_data.S" and "xsm/flask/flask-policy.S" without
having to list them a second time.

Also clean files in "arch/x86/boot" from that directory by allowing
"clean" to descend into the subdir by adding "boot" into $(subdir-).

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