fs/nvs: Switch to use flash_flatten instead of flash_erase

The NVS currently requires explict erase capability of
a device to work, so usage of flash_erase has been replaced
with flash_flatten.
There has been additional LOG_WRN added to warn user that
NVS may not efficiently work with device that do not really
have erase.

Currently NVS relies on devices that require erase.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>