Fixed fail to build from source due to a bad "&nbr->lladdr" reference
Small fixes to networking files

1. Added #include header
2. Fixed #defines that weren't updated
3. Small change to comment that now points to correct RFC section
Fixed a typo in net/uipopt.h
1. Added a missing '&' in lib/trickle-timer.h
2. Removed executable permissions of lib/trickle-timer.c & .h
Merge pull request #342 from zwuh/patch-trickle-lib

Typo and file mode fix for Trickle library, typo fix in uipopt.h
Merge pull request #341 from ismb-pert/fix-lladdr

Fixed fail to build from source due to a bad &nbr->lladdr
Add a way to determine whether a trickle timer is running
Only schedule a trickle interval doubling for running timers
Merge pull request #354 from lab11/core-net-fixes

Small fixes to networking files
Added SPI driver for CC2538

This uses the core/dev/spi.h header and implements the spi_init()
function and the various macros for SPI operation. ssi.h contains all of
the register locations and information.

This implementation is not very versatile, mostly because I don't how to
make it flexible in the contiki system. It supports pin muxing for the
four spi pins, but other than that picks sensible defaults.

The SPI macros (like SPI_READ()) are defined in
cpu/cc2538/spi-arch.h. In order to use the SPI driver, add the following
includes to your project:

    #include "spi-arch.h
    #include "dev/spi.h"
Merge pull request #352 from lab11/cc2538-spi-driver

Added SPI driver for CC2538
Use TRICKLE_TIMER_IS_STOPPED in trickle_timer_stop()
Merge pull request #359 from g-oikonomou/trickle-timer-stop

Add a method to stop running trickle timers