How to make phone boot automatically upon battery insertion (unlocked bootloader and root: yes)?
I soldered the pins of my Nexus 7 2012 Gen 1 and the device did start up. The soldering simulates a always pressed power on button.
However I tried the same with the Nexus 7 2013 Gen 2 and it turned itself off again, because the button was always pressed. So I needed to solder a capacitor and a resistor in series so simulate a short button press. This worked for me, but the software solution would be more elegant.
I could measure only a few uA when the battery is plugged in so I can't image a part of the bootloader is powering up on the mentioned devices. My idea is now to solder the power supply directly to the micro usb port as for data only 2 pins (USB2.0) are used. USB Pins: https://www.giga.de/downloads/microsoft-windows/tipps/usb-anschluss-farben-stecker-bedeutung-und-erklaerung/

Then the device will start up with the lpm script or the "Fastboot oem off-mode-charge 0" change.
An other solution would be really appreciated.
Thanks and Cheers