subsys: settings: fix fcb back-end initialization
subsys: settings: fix fcb back-end initialization

In f. settings_init_fcb the storage area was implemented badly:
- storage was always erased
- the only source of settings fcb instance was pointed twice.
This patch fix both issues.


Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

ldts/zephyr,zephyriot/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyriot/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,mbolivar/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,zephyriot/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,explora26/zephyr,punitvara/zephyr,kraj/zephyr,nashif/zephyr,zephyriot/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,mbolivar/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,kraj/zephyr,kraj/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr