WIP: core/msg: centralize message type declarations

TODO:
- [ ] convert remaining definitions

Remaining definitions:
```
git grep 'm.type =' | grep -v MT_TRANSCEIVER          
cpu/cc430/cc430-rtc.c:            m.type = RTC_SECOND;
cpu/x86/x86_rtc.c:        m.type = reg_c | (RTC_REG_B_INT_ALARM << 8);
cpu/x86/x86_rtc.c:        m.type = reg_c | (RTC_REG_B_INT_PERIODIC << 8);
cpu/x86/x86_rtc.c:        m.type = reg_c | (RTC_REG_B_INT_UPDATE << 8);
drivers/cc110x/cc1100_phy.c:        if (m.type == MSG_TIMER) {
drivers/cc110x/cc1100_phy.c:    m.type = MSG_POLL;
examples/ccn-lite-client/main.c:    m.type = CCNL_RIOT_CONFIG_CACHE;
examples/ccn-lite-client/main.c:    m.type = CCNL_RIOT_HALT;
examples/ccn-lite-client/main.c:        m.type = CCNL_RIOT_MSG;
examples/ccn-lite-client/main.c:    m.type = CCNL_RIOT_POPULATE;
examples/ccn-lite-client/main.c:    m.type = CCNL_RIOT_PRINT_STAT;
examples/ccn-lite-relay/main.c:    m.type = CCNL_RIOT_POPULATE;
examples/rpl_udp/helper.c:        else if (m.type == IPV6_PACKET_RECEIVED) {
sys/chardev_thread.c:            m.type = OPEN;
sys/net/ccn_lite/ccnl-ext-appserver.c:    m.type = CCNL_RIOT_MSG;
sys/net/ccn_lite/ccnl-riot-compat.c:    m.type = CCNL_RIOT_MSG;
sys/net/ccn_lite/ccnl-riot-compat.c:    m.type = CCNL_RIOT_NACK;
sys/net/ccn_lite/util/ccnl-riot-client.c:        m.type = CCNL_RIOT_MSG;
sys/net/ccn_lite/util/ccnl-riot-client.c:    m.type = CCNL_RIOT_MSG;
sys/net/ccn_lite/util/ccnl-riot-client.c:    m.type = CCNL_RIOT_MSG;
sys/net/transport_layer/socket_base/msg_help.c:    //  recv_m.type = TCP_NOT_DEFINED;
sys/posix/posix_io.c:    m.type = op;
sys/posix/posix_io.c:    m.type = op;
sys/uart0/uart0.c:    m.type = 0;
tests/queue_fairness/main.c:        m.type = i + 1;
```