Implement support for SAMD21 NVM controller

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.