arch/xtensa: accept the overlay to be an URL

It can be interesting to get the overlay from a remote server, rather
than expect it to be present locally.

Since that file can be any URL, we can't know its hash, so we just
exclude it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
[Thomas: use DL_DIR instead of BR2_DL_DIR.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>