drivers/mq3: added driver for alcohol sensor
tests: added test for MQ-3 alcohol sensor driver
board/stm32f4discovery: added ADC feature
Merge pull request #1703 from haukepetersen/add_driver_mq3

drivers/mq3: added driver for MQ-3 alcohol sensor
Enable SPI for udoo boards
Merge pull request #1897 from PeterKietzmann/add_udoo_spi

boards/udoo: added peripheral SPI definitions