travis: Add stm32 build in nanbox mode.
travis: Add stm32 build in nanbox mode.

MrSurly/micropython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,pramasoul/micropython,henriknelson/micropython,adafruit/circuitpython,kerneltask/micropython,tobbad/micropython,pozetroninc/micropython,MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,adafruit/circuitpython,tobbad/micropython,selste/micropython,adafruit/circuitpython,selste/micropython,pramasoul/micropython,pramasoul/micropython,kerneltask/micropython,MrSurly/micropython,MrSurly/micropython,tobbad/micropython,pozetroninc/micropython,pramasoul/micropython,kerneltask/micropython,selste/micropython,bvernoux/micropython,tobbad/micropython,pozetroninc/micropython,kerneltask/micropython,henriknelson/micropython,pozetroninc/micropython,adafruit/circuitpython,selste/micropython,bvernoux/micropython,selste/micropython,pozetroninc/micropython,adafruit/circuitpython,henriknelson/micropython,pramasoul/micropython,tobbad/micropython,henriknelson/micropython,adafruit/circuitpython,kerneltask/micropython