kbuild: buildtar: do not print successful message if tar returns error

The previous commit spotted that "Tarball successfully created ..."
is displayed even if the "tar" command returns error code because
it is followed by "| ${compress}".

Let the build fail instead of printing the successful message since
if the "tar" command fails, the output may not be what users expect.

Avoid the use of the pipe.  While we are here, refactor the script
removing the use of sub-shell, ${compress}, ${file_ext}.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>