Issue #384 : support all board channel banks update rates in GCS

Each boards maps output channels by banks (up to 6) for update rate settings.
GCS used hardcoded values in the config widget, all board-specific values are
now moved to board plugins, and the config gadget queries the board for the
mappings.

Please confirm mapping for Sparky is correct.
Merge pull request #718 from elafargue/channel_banks

Fixes Issue #384 : support all board channel banks update rates in GCS