ewaol-[config,distro,doc]: Move `meta-cassini-distro` into layers folder

These are the following changes:

* Point to the public repository of `meta-cassini`.
* Move `meta-cassini` into layers folder.
* Inherit `ewaol-sdk` distro feature from `cassini-sdk` distro
  feature from Cassini distro.
* Update documentation to reflect above changes.

cassini-[config,distro,tests,doc]: Remove `meta-cassini` folder

Since we are pointing to the public repository for the `meta-cassini`
distro which will be downloaded to the `layers` folder, therefore we no
longer need the changes in this repository.

Issue-Id: OSSEMBEDA-1749

Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>
Change-Id: I5e98e791d0394ee213bd34bc3f149832ce0fae23