Makefile: fix package file list if FOO_SUBDIR is set

Makefile: fix package file list if FOO_SUBDIR is set

If a package sets a FOO_SUBDIR (meaning its sources are not under
output/build/foo-123 but under output/build/foo-123/$(FOO_SUBDIR)), the
.files-list.txt file were also created under FOO_SUBDIR, due to which the
logic in the Makefile would not find it.

Change the instrumentation steps so that the file list is directly under the
package dir, ignoring the subdir.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>