Travis: Multiple Smaller Compile Builds and a Fix

This Pull Request splits travis compile tests down into sub-tests. The basic compile test, previously called 'compile' gets renamed to 'compile-base'. It now only includes {msp430, avr, native}-related tests. Tests relating to arm and 8051-based architectures have now been moved to their own builds (one per arch).

With this change in place, we can now only install arm-none-eabi, sdcc and srecord for the builds which actually need them. This will somewhat speed up all other tests.

The original compile test build had started growing too much and taking too long to complete. One positive side of having a larger number of smaller tests is that when we need to manually restart a test, the wait time for its completion will be shorter.

Other minor changes:
* We now also test er-rest-example and webserver-ipv6 for the cc2538
* An erroneous comment was fixed and another comment was somewhat clarified
* A cc2538dk test was building for the cc2530dk instead. This has been fixed.

It is now extremely easy to split compile tests to even smaller bundles. For instance, do we want to separate out sky tests to their own build?

This pull request fixes #247