stm32/mboot: Support boards with only two LEDs.
stm32/mboot: Support boards with only two LEDs.

Mboot currently requires at least three LEDs to display each of the four
states.  However, since there are only four possible states, the states can
be displayed via binary counting on only 2 LEDs (if only 2 are available).
The existing patterns are still used for 3 or 4 LEDs.

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