fs: Add FileSystem::reformat

[geky]

This function provides a shortcut to reformatting a mounted filesystem.

Usage without mounted filesystem:

BlockDevice *bd = user_provided;
int err = fs.mount(bd);
if (err) {
    fs.reformat(bd);
}
// fs is now mounted

Usage with mounted filesystem:

fs.reformat();
// fs is now mounted

Formatting without mounting

BlockDevice *bd = user_provided;
FATFileSystem::format(bd);
// fs is not mounted

Also, since this function is a virtual member of the FileSystem class, the user does not need to know the underlying filesystem to reformat the underlying storage.

can replace #4886
resolves #4903
related #4407
cc @netanelgonen, @sg-, @pan-, @theotherjimmy

[geky]

I'll go ahead and kick off CI to move things along
/morph test-nightly

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test-nightly

Output

mbed Build Number: 1046

All builds and test passed!

[geky]

I'm guessing this is ready to go in now. Last call for comments.

[JanneKiiskila]

Would it be possible to get this patched to mbed OS 5.5.x already?
@geky @adbridge

[theotherjimmy]

@JanneKiiskila We try to put only "bug fixes" into patch versions. This PR is queued for 5.6.0 because it adds a new API in a backwards compatible way. OTOH, it's been almost 3 months since the last minor release, so we're due for another one soon.