Fixed a problem for JN51xx in networks WITHOUT a PAN coordinator:
if no PAN coordinator is present, a infinite-loop causes the 802.15.4
radio (contiki-jn51xx/cpu/jennic/net/ieee802.c) to never finish its initialization, i.e. "ieee_started" is never
set to true.
Fixed this by introducing a maximum number of PAN searches.
Added support for the iSense JN5139 and JN5148 (UART, LED and radio work).