generate a CSP interface to get/send packets from AX25 layer

SP
/*
* 
*/
typedef struct {
    uint32_t src;
    uint32_t dest;
    uint32_t prio;
    uint32_t dport;
    uint32_t sport;
    uint32_t daddr;
    uint32_t saddr;
    uint32_t dn;
    uint32_t sn;
    uint8_t  cmd;
    uint8_t  data[CSP_BUFFER_DATA_LENGTH];
} csp_packet_t;

/* CSP interface for AX25 */
csp_iface_t csp_if_ax25 = {
    .name = "AX25",
    .nexthop = csp_ax25_tx,
};