scripts: kconfig: merge_config: config files: add a trailing newline

When merging files without trailing newlines at the end of the file, two
config fragments end up at the same row if file1.config doens't have a
trailing newline at the end of the file.

file1.config "CONFIG_1=y"
file2.config "CONFIG_2=y"
./scripts/kconfig/merge_config.sh -m .config file1.config file2.config

This will generate a .config looking like this.
cat .config
...
CONFIG_1=yCONFIG_2=y"

Making sure so we add a newline at the end of every config file that is
passed into the script.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>