package/nbd: fix utilities locations

Currently, the code expects both the client and server to be in
/usr/bin, while the client is in /usr/sbin

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>