utils/test-pkg: use the correct config prefix when merging

We should use an empty prefix as we do not have any prefix.
Note that BR2_ is mere a convention.

Signed-off-by: Nasser Afshin <Afshin.Nasser@gmail.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

utils/test-pkg

@@ -129,7 +129,7 @@ build_one() {
 
     mkdir -p "${dir}"
 
-    support/kconfig/merge_config.sh -O "${dir}" \
+    CONFIG_= support/kconfig/merge_config.sh -O "${dir}" \
         "${toolchainconfig}" "support/config-fragments/minimal.config" "${cfg}" \
         >> "${dir}/logfile" 2>&1
     # We want all the options from the snippet to be present as-is (set