pkg-download: support new subdirs for mirrors

pkg-download: support new subdirs for mirrors

Since we introduced subdirectories to the DL_DIR, we need to support them
in the PRIMARY and BACKUP mirrors as they evolve to the new tree
structure.

We check first the new URI (with the subdir), and in case of failure, we
check without. By checking both URIs, we ensure that old mirror are usable.

Also, add a missing qstrip call for BR2_BACKUP_SITE.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>