power: Add abstraction layer to configure voltage and toggle dut_vdd_en

[kbeckmann]

Maybe this module should control the shunt switches too. Need feedback from ADC module.

[kbeckmann]

@arturo182 What do you think of the following?

app/power.h:

err_t power_init(void);
err_t power_dut_enable(bool enabled);
err_t power_dut_ldo_set(uint32_t millivolt);

power_init:

• creates a task
• registers itself as an adc callback handler that posts adc values back to the task
• task then calculates current consumption and based on this flips the correct shunt resistor switches (in order to get a consistent voltage during load)
• it then sends batches of current values to a listener, i.e. to the USB endpoint. Not sure about the API, but I think it might make sense to batch up a few values.

The other two functions are pretty self explanatory.

[arturo182]

Sounds good except the power_dut_enable cause then you would get power_dut_enable(false), i think better would be power_dut_set_enabled