added init function in MAC driver for easier configuration

core/net/mac/ctdma_mac.c

core/net/mac/ctdma_mac.h

core/net/mac/cxmac.c

core/net/mac/cxmac.h

core/net/mac/lpp.c

core/net/mac/lpp.h

core/net/mac/mac.h

core/net/mac/nullmac.c

core/net/mac/sicslowmac.c

core/net/mac/tdma_mac.c

core/net/mac/xmac.c

platform/sky/contiki-conf.h

platform/sky/contiki-sky-main.c