esp8266 build unification

I finally unified the two esp8266 builds. Now there's only one :-) This changes the way the firmware is flashed on a 512KB module, the new instructions are at
https://github.com/tve/Espruino/blob/unify/targets/esp8266/README_flash.txt
and also in the downloaded tgz.

If you'd like to try it out, the firmware is now automatically built by travis and can be downloaded from
http://s3.voneicken.com/espruino/espruino_esp8266_1v81_tve_unify.tgz

The short form to flash this on linux to a 4MB module is:

curl -Ls http://s3.voneicken.com/espruino/espruino_esp8266_1v81_tve_unify.tgz | tar xfz -
cd espruino_esp8266_1v81_tve_unify
esptool.py --port [/dev/ttyUSB0|COM1] --baud 460800 write_flash \
  --flash_freq 80m --flash_mode qio --flash_size 32m \
  0x0000 "boot_v1.4(b1).bin" 0x1000 espruino_esp8266_user1.bin 0x37E000 blank.bin