Network protocol improvements

[matlo]

Instead of using a fixed-size byte array, allow to send packets in a more flexible format that allows to transmit changes only.

typedef struct {
    uint8_t packet_type; // E_NETWORK_PACKET_IN_REPORT
    uint8_t nbAxes; // number of elements in the following array
    struct {
        uint8_t index; // 1 MSB: 0 = relative axis, 1 = absolute axis, 7 LSB = index
        int32_t value; // network byte order
    } axes[0];
} s_network_report;