util-linux: Add an option to compile 'nsenter' binary.

nsenter is a tool to enter the namespace of another process.

[Thomas: add dependency to 3.0 kernel headers, since the setns()
system call was added in Linux 3.0.]

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>