base-files: sysupgrade: always cleanup after backups

base-files: sysupgrade: always cleanup after backups

When tar was failing, it was exiting immediately. Some files and the
tmpfs mount (-k) would remain breaking the next backup attempt.

Also remove redundant $? from exit builtin call as exit already returns
the last command exit code when called.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>