Add flash writer proposal.
Simplify flash_writer API.
Merge pull request #11 from nuft/flash_writer_proposal

Flash writer proposal
Add missing extern "C" statements
Convert data source adress type to void *

This should be the standard, as done for example in memcpy(3).
Add flash writing mocks
Rename .f to .callback in command parser
Add an output buffer argument to command execution
Add command output

Commands are now able to write to an output buffer. This allows the
command to give back informations to the client, such as application
CRC, etc.
Add config page flash operations
Initial implementation of the config serialization
Cleanup tests
Implement device class serialization
Update config keys in README

Now the README reflects the keys as they are stored in the flash.
Remove multicast groups config item

The multicast groups are not used by the protocol anymore, so they are
not needed.
Implement application CRC saving
Add CAN interface and mocks
Flash write page mock does a real write
Simplify flash mock
Implement flash writing command