Copy files from "native" platform to new "freertos" platform.
freertos: rename all occurrences of "native" to "freertos"
freertos: trim and adapt Makefiles
freertos: further cleanup and get it to compile (and run on "native" host)
platform/freertos/m: convenience script to invoke builds
platform/freertos (CONTIKI_SOURCEFILES): remove elfloader-stub.c

Doesn't seem to be used anywhere and it certainly won't exist on FreeRTOS.
cpu/freertos/: remove Windows and Apple code, leaving only Linux
freertos: replace Linux functions and headers with FreeRTOS counterparts (WIP)
freertos (contiki-conf.h, contiki-main.c): remove more Linux/POSIX-isms
freertos: add final target (hardware architecture); trim meta-arch Makefile