trace-cmd: new package

This commit adds a new package for the trace-cmd tool.  This tool is a
command line front end of ftrace.  It collects traces on your target.
You can analyse these traces on the target or on the host via the gui
"kernel shark".

[Thomas: use TARGET_CONFIGURE_OPTS, define _GNU_SOURCE to get
O_CLOEXEC definition on uClibc, add thread and largefile
dependencies.]

Signed-off-by: Pierre Floury <pierre.floury@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>