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>
(cherry picked from commit 2664022787)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

utils/test-pkg

@@ -100,7 +100,7 @@ build_one() {
 
     support/kconfig/merge_config.sh -O "${dir}" \
         "${toolchainconfig}" "support/config-fragments/minimal.config" "${cfg}" \
-        > /dev/null
+        >> "${dir}/logfile" 2>&1
     # We want all the options from the snippet to be present as-is (set
     # or not set) in the actual .config; if one of them is not, it means
     # some dependency from the toolchain or arch is not available, in