settings: zms: add more robustness to the save function

When a power off happens after writing the settings name and before
writing the linked list node we cannot write the settings name again in
the future.
Fix this by writing the linked list node before writing the name.
When loading all settings, we already delete linked list node that do
not have any name or value written.
Adds as well a recover path if a power down happens in the middle of
unlinking an LL node after a delete.

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>