With the introduction of BOARDSDIR external boards can re-use common
code of BOARDS present in RIOTBASE. To be able to do this file
references may not use $(BOARD) since BOARD will be set by the
external BOARD.
cpu/$(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.
PORT_LINUX and PORT_DARWIN are evaluated by
`makefiles/tools/serial.inc.mk` and some boards Makefile.include.
Their value does not need to be exported.
This also globally removes the line saying that the exports are needed
for flash rule.
