This commit cleans up the pin generation code, all the pins and their
AF (only ADC, for now) are specified in nrf52_af.csv and board use their
own csv file to specify which pins are available on that board and if
they have any special names.
With the only difference between a SD and non-SD build is the linker
file, there's no need to have two separate .mk per board files,
just use a conditional.
