Ultimately all static strings should be qstr. This entry in the type
structure is only used for printing error messages (to tell the type of
the bad argument), and printing objects that don't supply a .print method.
* Add simple ADC driver, with support for ADC1 and all channels.
* Export MicroPython ADC object with read_channel function.
* Add stm32f4xx_adc.c and adc.c to Makefile.