CC2538: Creating a base CC2538 platform

The better the Contiki support for the CC2538 gets, the more CC2538 based
hardware platforms that will be created. Right now, this is a bit tricky
as one must copy the cc2538dk platform and make a few changes for the
difference in hardware and then compile against the cpu/cc2538 folder.
This works until the cpu/cc2538 changes at which point platform/cc2538dk
gets updated but not the platforms sitting in other repositories and
compilation fails.

Because the changes to the cc2538dk platform to create a new platform
are usually small (typically initializing different hardware), we can
create a "base" cc2538 platform and build specific hardware platforms
off of that. This commit does just that. It spins most of the cc2538dk
platform into just a "cc2538" platform and uses the cc2538dk platform
for the /dev folder code and a couple of `init()` functions.

`contiki-main.c` now calls three new functions

- `platform_init_post_initial()`
- `platform_init_post_networking()`
- `platform_init_end()`

in the `main()` function. The init functions are called throughout
main() at places platform designers may want to add extra initialization
code. All init functions are defined in `platform.c`.

This commit also adds a `platform-conf.h` file that can be used to
override any of the defaults in `contiki-conf.h`.

Finally, this commit adds support for disabling the LEDs during boot.