Should have an easy way to update Mbed OS in exported project

I'm writing an application note for MDK and this is a real issue I encounter.

1. Find a project.
2. Export to MDK.
3. Now I want to update Mbed OS.

Because mbed-os is not under source control the normal `update`/`sync`/`deploy` things don't work. I need to:

1. Remove mbed and mbed-os folders.
2. Remove mbed.lib / mbed.bld / mbed-os.lib.
3. Run `mbed add mbed-os`.

Could we have some command that magically clears up all old Mbed folders and adds the latest?

Alternatively, a version of `mbed deploy` which will also work if the other folders are not under SCM could also work (but would need a big **WARNING** sign somewhere to avoid people losing data).