Rename adc_devinit() to board_adc_setup(). Add support to the boardctl()...

Rename adc_devinit() to board_adc_setup().  Add support to the boardctl() interface so that it can call board_adc_setup() on behalf of an application.  Change apps/examples/adc to that is now calls boardctl() instead of adc_devinit() in order to initalize the ADC device.