retention/retained memory: Mutex use cannot be de-selected by Kconfig

[nordicjm]

Describe the bug
Both retained memory and retention subsystem have Kconfig options to disable usage of mutexes, however because of how Kconfig works, these cannot be de-selected from other Kconfig options which is a requested feature. Therefore this should be reworked to support such a de-selection.

Expected behavior
To be able to de-select the options.

Impact
Unusable for some users

Environment (please complete the following information):

• Commit SHA or Version used: e1efafa