The setter function approach makes this switchable at runtime (antenna
diversity?).
Documentation is added both to the board introduction page and the
individual pins and functions.
The particle-{argon,boron,xenon} all have a uFL connector and a PCB
antenna for 2.4GHz radio, connected by a SKY13351-378LF switch
configured by a VCTL[12] pin pair.
This sets the default configuration to use the PCB antenna, as driving
an unconnected antenna will deteriorate radio performance.
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
