linux: add linux-tools infra

This commit add an infrastructure to build linux kernel
tools available in the kernel sources.

Currently, the only linux kernel tool packaged in Buildroot
is perf and it's packaged as a separate generic package.
This is a problem for licence information raised in this
thread [1].

Since these tools require to build a Linux kernel, we can
use some hooks in linux package like we did for linux
extensions [2] and remove the perf package.

[1] http://lists.busybox.net/pipermail/buildroot/2015-May/128783.html
[2] http://lists.busybox.net/pipermail/buildroot/2015-March/121835.html



[Thomas: fix minor typos in comments.]

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