SBP protocol versioning

[mfine]

Bring the SBP protocol versioning into piksi_tools, piksi_firmware, piksi_bootloader.

[mfine]

Success criteria: getting SBP protocol versioning into libsbp (pending), piksi_tools, piksi_firmware, and piksi_bootloader.

[mfine]

The changes for SBP protocol versioning are merged into libsbp. Pending changes are all in:

• piksi_bootloader new bootloader msgs piksi_bootloader#9
• piksi_firmware New libsbp and new bootloader messages piksi_firmware#416
• piksi_tools Versioned bootloader piksi_tools#57

/cc @cbeighley