utils/test-pkg: log the output of merge-config

utils/test-pkg: log the output of merge-config

Currently, the stdout is consigned to oblivion, while the stderr is not
redirected at all. So, when the configuration under test redefines a
symbol, like is the case when testing busybox for example, there is an
ugly warning.

So, just redirect both stdout and stderr to the logfile.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>