Break travis compile tests into smaller builds

* The previous 01-compile test has been renamed to compile-base. It
  now only tests {msp430, avr, native}-based platforms
* New test builds for arm-based and 8051-based ports (one build per arch)
* Only install arm-none-eabi, sdcc and srecord for the builds which need
  them. This will somewhat speed up all other tests
* Test er-rest-example and webserver-ipv6 for the cc2538
* An erroneous comment was fixed and another comment was somewhat
  clarified
Fix travis string comparisons