Improvements to codeplug data structures

[silseva]

1. Change the magic field of the header to uint32_t, as four bytes are sufficient to hold the identification string.
2. Extend the version field to uint32_t to provide space for the full major-minor-patch triplet.
3. Use an uint32_t to store the transmission power of each channel, expressed in mW. Max storable value is a bit less than 4.3MW.
4. Set the size of the channel infoblock field to a fixed value, like 16 bytes, to make room for future expansions without needing to resize the codeplug file (good when stored without a filesystem).
5. Use a single CAN in M17 channels, as there is no need for separate TX/RX CAN.
6. Compact the mode_encr and gps_mode fields of M17 channels into a single 8-bit field.

[silseva]

1. done
2. done
3. done
4. to be done
5. to be done
6. to be done