fs: Add support for mount flags

The flags field has been added to fs_mount_t structure, accompanied
with two new flags:
  FS_MOUNT_FLAG_READ_ONLY -- mount fs as read only
  FS_MOUNT_FLAG_NO_FORMAT -- do not format volume when system not found

Code supporting the flags has been added to FS layer and drivers for
LittleFS and FAT FS.

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