timex: unambiguous time conversion macros

cpu/cc2538/periph/i2c.c

cpu/lpc2387/mci/lpc2387-mci.c

drivers/dht/dht.c

drivers/servo/servo.c

drivers/si70xx/si70xx.c

drivers/xbee/xbee.c

examples/timer_periodic_wakeup/main.c

pkg/emb6/contrib/target.c

pkg/lwip/contrib/sock/lwip_sock.c

pkg/lwip/contrib/sys_arch.c

sys/include/net/gnrc/ndp.h

sys/include/net/gnrc/sixlowpan/nd.h

sys/include/net/gnrc/tftp.h

sys/include/net/sock/ip.h

sys/include/net/sock/udp.h

sys/include/timex.h

sys/include/xtimer/implementation.h

sys/net/application_layer/sntp/sntp.c

sys/net/application_layer/uhcp/uhcpc.c

sys/net/gnrc/application_layer/tftp/gnrc_tftp.c