Implement support for SAMD21

> Implement support for SAMD21
> Add beforeWrite callback to Flash class, so that operations like erase can be done easily
> Add bufferSize as payload to Flash::loadBuffer callback. This is needed to handle partial writes gently.
> Handle detection of SAMD21 in Samba. There is no deterministic way to do that, so used trial and error method to do so.
> Updated wxWidgets version, please advise if that should be reverted back.