CC2538 IEEE address and README updates

TI has now released a new version of the SmartRF Flash Programmer, adding support for the CC2538 SoC.

Crucially, the new Flash Programmer allows us to manipulate the CC2538 InfoPage. Thus, we can now burn a new IEEE address to nodes if they don't have a factory-provided one. As a result, we no longer need to use a MAC address hard-coded in contiki (`ieee-addr.[ch]` and `IEEE_ADDR_CONF_HARDCODED`). This Pull Request changes the default behaviour to use the IEEE address from InfoPage by default (Previous default was to use the contiki hard-coded one). Support for hard-coding remains. Support for reading the address from a secondary location to be added soon (which probably means I shouldn't have removed it in the first place... :))

The platform's README has been updated to reflect the new tool release, as well as to include some more links to user documentation.