fix(serial): Unblock if the queue is full

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

epicardium/modules/serial.c

@@ -119,8 +119,12 @@ void epic_uart_write_str(const char *str, size_t length)
 					SERIAL_WRITE_NOTIFY,
 					eSetBits
 				);
+				if (bytes_sent == 0) {
+					vTaskDelay(1);
+				} else {
 					portYIELD();
 				}
+			}
 		} while (index < length);
 	}
 }