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.
The following features were tested (briefly):
* UART (the first)
* I2C using a BMP180 on a SODAQ TPH board
* SPI0, connected to the on-board serial data flash (only DevID was read
using the periph_spi test program)
* xtimer is working (it's the same code as in samr21)
boards/sodaq-autonomo: Disable the "big" unittests because it does not fit
