Allow attribute get on root

[qistoph]

This PR enables attributes on the root, such as creation/modification time of volume.

  fs::Dir fs = LittleFS.openDir("/");
  Serial.print("FS creation timestamp: ");
  Serial.println(fs.fileCreationTime());

After quite some testing I believe the attached changed is the only one required. It didn't seem to break anything in my tests, though I'm not 100% sure, because the use and value of _valid in undocumented and unclear to me.

Test project available: LittleFSTest

Effects of checking 'c' and 't' attributes on a root node, if:

1. attribute present: return its value (mklittlefs since PR15)
2. attribute unavailable at root and sub nodes: return 0 (older version of mklittlefs)
3. attribute unavailable at root, but available at sub node: return (last?) sub node's value

I'm haven't been able to figure out why situation 3. doesn't respond like 2. If this is an undesirable side effect, I'm hoping someone has a suggestion on fixing it or where to look.

[earlephilhower]

I'm worried that when _valid is false the filename is not guaranteed to be proper. Has this been verified?

Also, the ::format method should be setting these attributes when on-device formatting occurs.

[qistoph]

I'll extend my tests and verify the workings of all available methods.

Where would you say is the right place to set the attributes on ::format?

1. LittleFSImpl::format (LittleFS.h)
2. lfs_format (lfs.c)

The first would be most in line with the change in mklittlefs, I guess.

[earlephilhower]

The LittleFSImpl::format() would be the spot to add the attributes. We don't touch the upstream FS lib itself.

The _valid thing still sets off alarms. You're assuming that dirent is all 0'd (which it is, luckily, in the constructor...but only until something writes to it which can happen on rewind() or next() or other calls). Also, this would return the FS date no matter what the subdir it's in which is not what you're going for. Currently, also, if I read the sprintf logic you're doing a lfs_get_attr("") and not lfs_get_attr("/")

[qistoph]

Thanks for the feedback and insights. I have updated the code to a more proper way to get the attributes. Also included is the proposed change to set the attributes on format.

LittleFS.creationTime()

[earlephilhower]

Good way to add in the call! Just some minor things and we should be good to go!