Only set inodes for the root filesystem when we're not using a block device

This fixes #1845 (closed) without breaking #1717 (closed). The amount of inodes only needs to be explicitly set when we don't know the size of the filesystem before hand, for example when using fastboot or Heimdall. In all other cases it's actually better to let it figure out automatically how many inodes is set.

On my 16GB sdcard the amount of available inodes got increased by 10 times (from ~100K to ~1M).