infra: fix "<pkg>-rebuild" when using rsync source feature

Because the configure stamp file depends on the rsync stamp file,
removing it by the "<pkg>-clean-for-rebuild" rule trigger the configure
step. To avoid triggering the configure step use an order-only dependency.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reported-by: Cédric Marie <cedric.marie@openmailbox.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>