This is more than just WIP. hetii has started a serprog implementation on the ESP8266 and Stefanct will set up test hardware if that's ready. Till then this will host various ESP-related bits not necessarily related to Serprog.
The default UART requires line feeds to be sent. This can be done by ending every line with '<enter> <ctrl>-j'. A more convenient solution is to use automatic mapping provided by a terminal emulator, e.g.:
picocom -b 115200 --omap crcrlf /dev/ttyUSB1
After boot you should see (at least) the ready message as below. You can test if the UART connection works correctly by issuing AT commands.
Ai-Thinker Technology Co.,Ltd. ready AT OK AT+GMR AT version:0.40.0.0(Aug 8 2015 14:45:58) SDK version:1.3.0 Ai-Thinker Technology Co.,Ltd. Build:184.108.40.206 Sep 11 2015 11:48:04 OK