fix(serial): Unblock if the queue is full

schneider requested to merge schneider/218-tty-fix into master

For some reason a portYIELD() is not enough to give the serial task the option to run. I tried vTaskDelay() and that works. To not block more than needed portYIELD() is still called when the queue is not full and only call vTaskDelay() when it is full.

Closes #218 (closed)

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