subsys: settings: Fix Kconfig dependencies

If CONFIG_FLASH_PAGE_LAYOUT is not enabled, building the settings code
fails at the linking stage with the following error when FCB is the
backend.

libzephyr.a(settings_init.c.obj): In function `settings_init_fcb':
subsys/settings/src/settings_init.c:62: undefined reference to `flash_area_get_sectors'
collect2: error: ld returned 1 exit status

Also, the File System backend will currently only work with NFFS due
to fs_rename() missing from other File Systems (FAT in particular).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>