Makefile.freertos, m: introduce CONTIKI_TARGET_CPU to select the CPU we build for

The use of "CPU" is a bit confusing. We have "native" and now "freertos"
CPU's, which should probably better be called "architectures". And then
we have the actual CPU (instruction set and such) we build for.
CONTIKI_TARGET_CPU is the latter, when the architecture is "freertos".