tools: cell-list: make python3 compatible

Avoid python2 only functions.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
tools: config-create: set default root cell name to RootCell

The cell name could end up as an empty string because it was derived
from optional input files. In fact just giving the root cell a fixed
default name seems to make more sense than to generate a name or require
users to provide one.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
tools: config-create: close output file before exiting from script

Maybe not strictly required in that case, but open should always be
followed by close.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>