Merge pull request #4315 from haukepetersen/doc_periph_fixes

cpu/kinetis_common/include/periph_cpu.h

cpu/samd21/include/periph_cpu.h

doc/doxygen/src/mainpage.md

drivers/include/adc_legacy.h

drivers/include/periph/adc.h

drivers/include/periph/cpuid.h

drivers/include/periph/dac.h

drivers/include/periph/dev_enums.h

drivers/include/periph/doc.txt

drivers/include/periph/gpio.h

drivers/include/periph/i2c.h

drivers/include/periph/pwm.h

drivers/include/periph/random.h

drivers/include/periph/rtc.h

drivers/include/periph/rtt.h

drivers/include/periph/spi.h

drivers/include/periph/timer.h

drivers/include/periph/uart.h

sys/include/net/gnrc/slip.h