Massive platform cleanup

This is a big pull request that removes a bunch of platforms and that might need a bit of discussion before we merge it, as we need to figure out if these platforms really should go or not. The platforms that are removed in this pull request are:

* `esb`: this is an old custom MSP430-based platform for which no hardware currently exists (in my knowledge), but since this one of the early MSP430-based Contiki platforms it has a fairly substantial support in Contiki such as its own Cooja emulation platform. But since nobody uses it, we should just drop it.
* `iris`: I'm not sure what hardware this corresponds to, but it is an avr-based platform according to the Makefile. It does not have any commits besides maintenance commits since two years back.
* `jcreate`: this was a MSP430-based platform that was similar to the sky platform, but with a nice form factor and a plastic casing which made it easy to handle. The hardware isn't available anymore.
* `msb430`: this is a custom MSP430-based platform that I don't this is available anymore.
* `pc-6001`: this is one of the homecomputer platforms that, while useful since it is the last z80 platform, isn't maintained anymore. (It is a shame to drop z80 support though :cry:)
* `redbee-dev` and `redbee-econotag`:  I *think* these are obsolete by now, but @malvira will have to weigh on on this.
* `sentilla-usb`: this is a relative to the `jcreate` platform, a USB-device that had a MSP430 and a CC2420 radio. The hardware is not available anymore.
* `stepper-robot`: this is an AVR-based platform that I think is a custom built hardware. It is not maintained and I don't think there is hardware available.
* `stk500` and `stk501`: these are AVR-based platforms for which I think there may be hardware around, but that seem like they are obsolete. Someone with more AVR wisdom needs to comment on this.

In general, I think all platforms in the tree should have corresponding travis regression tests so that we can ensure that everything is up to date at all times.