base-files: sysupgrade: do not hide tar errors

base-files: sysupgrade: do not hide tar errors

tar stderr was probably discarded only to remove this message:

  tar: removing leading '/' from member names

However, together with that, any other error would also be discarded.
It is easier to fix that allowing the error message to be printed.

In sysupgrade, the backup file list only uses absolute paths. That way,
the solution is to remove the leading '/' from all files (sed) and chdir
to / (option -C /)

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