after some help on Github I was able to flash and use my ESP8266-01 via USB2TTL connector (switch at 3V3), see:
Now I tried the same cabling with ESP8266-12 and got continuous resets (seen in clear via minicom @115200).
I measured exactly 3.00V between GND and VCC on ESP8266-12 which seems to be too low.
Next I added LD33V into the mix and switched USB2TTL to 5V, resulting in 3.28V between GND and VCC on ESP8266-12, again with continuous resets.
Finally I removed LD33V and directly used USB2TTL with switch at 5V, resulting in 4.88V between GND and VCC on ESP8266-12, and this time its working?!?!?
Now I am not sure whether running ESP8266-12 at 4.88V over USB port is a good idea, thoughts?