Temporarily update lib/esp-open-rtos dependency to use forked version

I extended the `esp-open-rtos/extras/MAX7219` software driver and pushed
my changes upstream for merge into the main repository. Until the change
is merged, this project will use my fork of the dependency.

See [SuperHouse/esp-open-rtos PR #765](SuperHouse/esp-open-rtos#765)
for the proposed upstream changes.

The forked esp-open-rtos dependency adds software driver support for
bitmap drawing to cascaded 8x8 LED Matrix displays using the MAX7219
hardware driver.