package/linux-tools: don't register each tools twice

Because each tool's individual .mk files is included from the top-level
Makefile and we also include them from linux-tools.mk, they get
registered twice, and thus built twice, and thus installed twice.

We did include them from linux-tools.mk to guarantee they would be
included early and each tool had a chance to register itself before we
were to construct the build and install hooks.

However, the ordering is _currently_ guaranteed, in the C locale by the
files names, which we anyway sort using make's $(sort) function, which
always sorts in the C locale.

Beware if we are to ever rename those files in the future...

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