enable USB serial gadget mode by default

[weberjn]

USB serial gadget mode should enabled by default on freshly written raspbian images.
If this is not possible, add a control file mechanism (like /boot/ssh ) to enable it for headless devices.
It should be possible to get a login prompt in putty in usb mode in a fresh system, without the need for lan or wifi.
For a Zero W 2 I followed this tutorial from a running network enabled system.
This should be enabled by default or by touching a control file in /boot from a Windows laptop. The user should then be able to plug a pi into the laptop usb port and get a login prompt in putty.

Benefits for headless devices:

• device can immediately be accessed without the need for a working network
• if you misconfigure networking you can repair it from the usb console

Alternatives:

• keyboard and display, but still no copy/paste from the internet
• find a computer with an sd reader, that can write ext4 and repair on the mounted sd