buildroot

traceroute: pass INSTALL=$(INSTALL) as install time

As reported by Peter, using cp during the installation isn't correct,
as it follows symbolic links, potentially over-writing the busybox
binary through its traceroute symbolic link. To avoid that, we pass
INSTALL=$(INSTALL) during the installation step.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>