Ok, however, let's imagine the following situation: I boot it up, it's the first boot. It checks the nvs for a certain value, it does not find it. It sets up that value saying that the next boot isn't the first. After the deepsleep, it will know that it isn't the first boot. But, if I reset the board, it will still think that it's not the first boot. How can I get around this ?

@rskoniec Thanks, it helped debugging. If I unplug the power completely it shows the POWER_ON_WAKE, but if I do a manual reset (button pressed) it says TIMER_WAKE. I reckon it's a bit confusing. I couldn't see the values after connecting the USB and reconnecting to the console, so I assumed a manual reset would be treated as a POWER_ON_WAKE.