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.
This adds the port parameter to the stm32loader flash script. This was
necessary to allow flashing on macOS X where the default port is not
autodetected correctly and needs to be specified, e.g. by running:
BOARD=lobaro-lorabox PORT=/dev/tty.SLAB_USBtoUART \
make -C examples/lorawan flash term`