In order to flash new firmware into ESP-12. The device must enter bootloader mode. If GPIO 0 and GPIO 15 are pulled low and GPIO 2 is pulled high during reset (or power on). The device enters bootloader mode.
On PC I used the following command to load the LUA interpreter.
esptool.py -p /dev/ttyUSB0 write_flash -fm dio -fs 32m 0x00000 nodemcu_float_0.9.6-dev_20150704.bin
After that the interpreter can be accessed via the serial line. The baudrate is 9600 baud.
This little blinky example works.
Maybe coding in LUA on the ESP-12 is more effective than coding in directly in C or using micropython.
There are some useful examples.