Implement block transfer for Static API

[datachi7d]

@DavidAntliff and I propose to implement block transfer support for the Static API. We consider this to be a special case of an opaque resource write operation.

• What we are trying to solve
  • Write only transfer of a large opaque resource (example firmware object's package resource /5/0/0).
  • Large means bigger than a coap packet allows, therefore requires coap block transfer.
  • Avoid buffering entire resource (maybe too large for memory).
  • Could write blocks to file.
• How we propose to solve it
  • Consider opaque block resource as special case:
    • Can't be transferred as part of a whole object (block transfer failures are hard to handle without buffering).
    • Can only write resource on its own and not as part of an object write.
• Erbium support only and no libcoap.
• Actions:
  • Investigate current status of block transfer and coap standard.
  • Investigate and test erbium support for block transfer
  • Extend Static API to support opaque block resource and offer for comments.
  • Implementation and tests.

[datachi7d]

Relates to #292