vmtest: add s390x config

Select the current config based on $ARCH value and thus rename
the existing config to config-latest.$ARCH.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/vmtest/helpers.sh

@@ -22,3 +22,5 @@ travis_fold() {
     echo -e "$line"
   fi
 }
+
+ARCH=$(uname -m)

travis-ci/vmtest/prepare_selftests.sh

@@ -14,7 +14,7 @@ fi
 if [[ "${KERNEL}" = 'LATEST' ]]; then
 	travis_fold start build_kernel "Kernel build"
 
-	cp ${VMTEST_ROOT}/configs/latest.config .config
+	cp "$VMTEST_ROOT"/configs/config-latest."$ARCH" .config
 	make -j $((4*$(nproc))) olddefconfig all >/dev/null
 	travis_fold end build_kernel
 fi